I have the same problem. When running the updater I get the same output as @solarPowerPlant.
Here is the relevant content of my Mariadb error.log:
> 2022-06-02 9:25:05 0 [Note] Recovering after a crash using tc.log
> 2022-06-02 9:25:05 0 [Note] Starting crash recovery...
> 2022-06-02 9:25:05 0 [Note] Crash recovery finished.
> 2022-06-02 9:25:05 0 [Note] Server socket created on IP: '127.0.0.1'.
> 2022-06-02 9:25:05 0 [Note] Reading of all Master_info entries succeeded
> 2022-06-02 9:25:05 0 [Note] Added new Master_info '' to hash table
> 2022-06-02 9:25:05 0 [Note] /usr/sbin/mysqld: ready for connections.
> Version: '10.3.34-MariaDB-0ubuntu0.20.04.1' socket: '/run/mysqld/mysqld.sock' port: 3306 Ubuntu 20.04
> 2022-06-02 9:25:05 0 [Note] InnoDB: Buffer pool(s) load completed at 220602 9:25:05
> 2022-06-02 9:28:25 61 [Note] InnoDB: Ignoring strange row from mysql.innodb_index_stats WHERE database_name = 'nextcloud' AND table_name = 'oc_oauth2_clients' AND index_name = 'oauth2_client_id_idx' AND stat_name = 'n_diff_pfx02'; because stat_name is out of range, the index has 1 unique columns
> 2022-06-02 09:28:26 0x7f8f84376700 InnoDB: Assertion failure in file /build/mariadb-10.3-AwHSs6/mariadb-10.3-10.3.34/storage/innobase/dict/dict0mem.cc line 143
> InnoDB: Failing assertion: dict_tf2_is_valid(flags, flags2)
> InnoDB: We intentionally generate a memory trap.
> InnoDB: Submit a detailed bug report to https://jira.mariadb.org/
> InnoDB: If you get repeated assertion failures or crashes, even
> InnoDB: immediately after the mysqld startup, there may be
> InnoDB: corruption in the InnoDB tablespace. Please refer to
> InnoDB: https://mariadb.com/kb/en/library/innodb-recovery-modes/
> InnoDB: about forcing recovery.
> 220602 9:28:26 [ERROR] mysqld got signal 6 ;
> This could be because you hit a bug. It is also possible that this binary
> or one of the libraries it was linked against is corrupt, improperly built,
> or misconfigured. This error can also be caused by malfunctioning hardware.
>
> To report this bug, see https://mariadb.com/kb/en/reporting-bugs
>
> We will try our best to scrape up some info that will hopefully help
> diagnose the problem, but since we have already crashed,
> something is definitely wrong and this may fail.
>
> Server version: 10.3.34-MariaDB-0ubuntu0.20.04.1
> key_buffer_size=134217728
> read_buffer_size=131072
> max_used_connections=4
> max_threads=153
> thread_count=9
> It is possible that mysqld could use up to
> key_buffer_size + (read_buffer_size + sort_buffer_size)*max_threads = 467428 K bytes of memory
> Hope that's ok; if not, decrease some variables in the equation.
>
> Thread pointer: 0x7f8f340015c8
> Attempting backtrace. You can use the following information to find out
> where mysqld died. If you see no messages after this, something went
> terribly wrong...
> stack_bottom = 0x7f8f84375dd8 thread_stack 0x49000
> /usr/sbin/mysqld(my_print_stacktrace+0x32)[0x564681897b92]
> /usr/sbin/mysqld(handle_fatal_signal+0x55d)[0x56468138d1bd]
> /lib/x86_64-linux-gnu/libpthread.so.0(+0x14420)[0x7f8f9bfe1420]
> /lib/x86_64-linux-gnu/libc.so.6(gsignal+0xcb)[0x7f8f9be1e00b]
> /lib/x86_64-linux-gnu/libc.so.6(abort+0x12b)[0x7f8f9bdfd859]
> /usr/sbin/mysqld(+0x4ee4bd)[0x5646810a94bd]
> /usr/sbin/mysqld(+0x4ffa8f)[0x5646810baa8f]
> /usr/sbin/mysqld(+0x987a86)[0x564681542a86]
> /usr/sbin/mysqld(+0x98c70a)[0x56468154770a]
> /usr/sbin/mysqld(+0x6769a8)[0x5646812319a8]
> /usr/sbin/mysqld(_Z17mysql_alter_tableP3THDPK25st_mysql_const_lex_stringS3_P14HA_CREATE_INFOP10TABLE_LISTP10Alter_infojP8st_orderb+0x3720)[0x56468123c870]
> /usr/sbin/mysqld(_ZN19Sql_cmd_alter_table7executeEP3THD+0x2ba)[0x56468128f62a]
> /usr/sbin/mysqld(_Z21mysql_execute_commandP3THD+0xdc8)[0x56468119dd28]
> /usr/sbin/mysqld(_Z11mysql_parseP3THDPcjP12Parser_statebb+0x1f3)[0x5646811a5053]
> /usr/sbin/mysqld(_Z16dispatch_command19enum_server_commandP3THDPcjbb+0x1065)[0x5646811a7425]
> /usr/sbin/mysqld(_Z10do_commandP3THD+0x12d)[0x5646811a93dd]
> /usr/sbin/mysqld(_Z24do_handle_one_connectionP7CONNECT+0x1fa)[0x56468128c9ba]
> /usr/sbin/mysqld(handle_one_connection+0x3f)[0x56468128cb8f]
> /lib/x86_64-linux-gnu/libpthread.so.0(+0x8609)[0x7f8f9bfd5609]
> /lib/x86_64-linux-gnu/libc.so.6(clone+0x43)[0x7f8f9befa133]
>
> Trying to get some variables.
> Some pointers may be invalid and cause the dump to abort.
> Query (0x7f8f3400fff0): ALTER TABLE oc_mounts ADD mount_provider_class VARCHAR(128) DEFAULT NULL
>
> Connection ID (thread ID): 61
> Status: NOT_KILLED
>
> Optimizer switch: index_merge=on,index_merge_union=on,index_merge_sort_union=on,index_merge_intersection=on,index_merge_sort_intersection=off,engine_condition_pushdown=off,index_condition_pushdown=on,derived_merge=on,derived_with_keys=on,firstmatch=on,loosescan=on,materialization=on,in_to_exists=on,semijoin=on,partial_match_rowid_merge=on,partial_match_table_scan=on,subquery_cache=on,mrr=off,mrr_cost_based=off,mrr_sort_keys=off,outer_join_with_cache=on,semijoin_with_cache=on,join_cache_incremental=on,join_cache_hashed=on,join_cache_bka=on,optimize_join_buffer_size=off,table_elimination=on,extended_keys=on,exists_to_in=on,orderby_uses_equalities=on,condition_pushdown_for_derived=on,split_materialized=on
>
> The manual page at https://mariadb.com/kb/en/how-to-produce-a-full-stack-trace-for-mysqld/ contains
> information that should help you find out what is causing the crash.
> Writing a core file...
> Working directory at /var/lib/mysql
> Resource Limits:
> Limit Soft Limit Hard Limit Units
> Max cpu time unlimited unlimited seconds
> Max file size unlimited unlimited bytes
> Max data size unlimited unlimited bytes
> Max stack size 8388608 unlimited bytes
> Max core file size 0 unlimited bytes
> Max resident set unlimited unlimited bytes
> Max processes 7626 7626 processes
> Max open files 32768 32768 files
> Max locked memory 65536 65536 bytes
> Max address space unlimited unlimited bytes
> Max file locks unlimited unlimited locks
> Max pending signals 7626 7626 signals
> Max msgqueue size 819200 819200 bytes
> Max nice priority 0 0
> Max realtime priority 0 0
> Max realtime timeout unlimited unlimited us
> Core pattern: core
>
> 2022-06-02 9:28:32 0 [Warning] The parameter innodb_file_format is deprecated and has no effect. It may be removed in future releases. See https://mariadb.com/kb/en/library/xtradbinnodb-file-format/
> 2022-06-02 9:28:32 0 [Note] InnoDB: Using Linux native AIO
> 2022-06-02 9:28:32 0 [Note] InnoDB: Mutexes and rw_locks use GCC atomic builtins
> 2022-06-02 9:28:32 0 [Note] InnoDB: Uses event mutexes
> 2022-06-02 9:28:32 0 [Note] InnoDB: Compressed tables use zlib 1.2.11
> 2022-06-02 9:28:32 0 [Note] InnoDB: Number of pools: 1
> 2022-06-02 9:28:32 0 [Note] InnoDB: Using SSE2 crc32 instructions
> 2022-06-02 9:28:32 0 [Note] InnoDB: Initializing buffer pool, total size = 128M, instances = 1, chunk size = 128M
> 2022-06-02 9:28:32 0 [Note] InnoDB: Completed initialization of buffer pool
> 2022-06-02 9:28:32 0 [Note] InnoDB: If the mysqld execution user is authorized, page cleaner thread priority can be changed. See the man page of setpriority().
> 2022-06-02 9:28:32 0 [Note] InnoDB: Starting crash recovery from checkpoint LSN=302577015045
> 2022-06-02 9:28:32 0 [Note] InnoDB: Starting final batch to recover 3 pages from redo log.
> 2022-06-02 9:28:32 0 [Note] InnoDB: 128 out of 128 rollback segments are active.
> 2022-06-02 9:28:32 0 [Note] InnoDB: Removed temporary tablespace data file: "ibtmp1"
> 2022-06-02 9:28:32 0 [Note] InnoDB: Creating shared tablespace for temporary tables
> 2022-06-02 9:28:32 0 [Note] InnoDB: Setting file './ibtmp1' size to 12 MB. Physically writing the file full; Please wait ...
> 2022-06-02 9:28:32 0 [Note] InnoDB: File './ibtmp1' size is now 12 MB.
> 2022-06-02 9:28:32 0 [Note] InnoDB: 10.3.34 started; log sequence number 302577015526; transaction id 1347040659
> 2022-06-02 9:28:32 0 [Note] InnoDB: Loading buffer pool(s) from /var/lib/mysql/ib_buffer_pool
> 2022-06-02 9:28:32 0 [Note] Plugin 'FEEDBACK' is disabled.
> 2022-06-02 9:28:32 0 [Note] Recovering after a crash using tc.log
> 2022-06-02 9:28:32 0 [Note] Starting crash recovery...
> 2022-06-02 9:28:32 0 [Note] Crash recovery finished.
> 2022-06-02 9:28:32 0 [Note] Server socket created on IP: '127.0.0.1'.
> 2022-06-02 9:28:33 0 [Note] Reading of all Master_info entries succeeded
> 2022-06-02 9:28:33 0 [Note] Added new Master_info '' to hash table
> 2022-06-02 9:28:33 0 [Note] /usr/sbin/mysqld: ready for connections.
> Version: '10.3.34-MariaDB-0ubuntu0.20.04.1' socket: '/run/mysqld/mysqld.sock' port: 3306 Ubuntu 20.04
> 2022-06-02 9:28:33 0 [Note] InnoDB: Buffer pool(s) load completed at 220602 9:28:33
] Check for expected files
Thank you so much!!!