Had the same issue, and after putting related pieces together it seems that in my case MariaDB 10.3 was running out of memory. Running mysqlcheck in the optimize option seemed to have resolved the issue. You will get “Table does not support optimize, doing recreate + analyze instead” for each table it runs on which you can ignore (Informal message for InnoDB tables - just let it finish)
sudo mysqlcheck -o nextcloud
I had also gone back into my MariaDB configuration file and updated some of the memory values which hadn’t been touched since I first installed several years ago (not sure if this did anything):
key_buffer_size = 64K
max_allowed_packet = 16M
thread_stack = 299K
thread_cache_size = 256
This was the hint: https://help.nextcloud.com/t/solved-sqlstate-hy000-general-error-2006-mysql-server-has-gone-away/22680