SMB/CIFS mount not working on 16.0.6 or 17.0.1, works on 15.0.13

the following problem happened after an upgrade from 15.0.13 to 16.0.6:

New external storages, SMB/CIFS mounts, can’t be made

Nextcloud version: 16.0.6
Operating system and version : debian buster 10.2
Apache version: 2.4.38-3+deb10u3
PHP version :7.3

The issue you are facing:

New external storages, SMB/CIFS mounts, can’t be made

Is this the first time you’ve seen this error? :Y

Steps to replicate it:

  1. go to Administration / External Storage
  2. check: Allow users to mount external storage
  3. check: SMB / CIFS
  4. go to Personal / External Storages
  5. (try) add Personal Storage: red icon (!), no mounting possible.

log_level is debug, but no error is shown:
The output of your Nextcloud log in Admin > Logging:

[user_ldap] Debug: Ready for a paged search

GET /nextcloud/index.php/settings/admin/logging
from 129.27.3.13 by xxxx at 2019-11-27T07:42:44+00:00

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):

    <?php
    $CONFIG = array (
      'loglevel' => 0,
      'instanceid' => 'xxxxxxxxxx',
      'passwordsalt' => 'xxxxxxxxxxxx',
      'secret' => 'xxxxxxxxxx',
      'trusted_domains' =>
      array (
        0 => 'xxxxxxx-dev.tugraz.at',
      ),
      'datadirectory' => '/srv/data',
      'dbtype' => 'mysql',
      'version' => '16.0.6.1',
      'overwrite.cli.url' => 'https://xxxxxxx-dev.tugraz.at/nextcloud',
      'dbname' => 'nextcloud',
      'dbhost' => 'localhost',
      'dbport' => '',
      'dbtableprefix' => 'oc_',
      'mysql.utf8mb4' => true,
      'dbuser' => 'nextcloud',
      'dbpassword' => 'xxxxxxxx',
      'installed' => true,
      'maintenance' => false,
      'ldapIgnoreNamingRules' => false,
      'ldapProviderFactory' => 'OCA\\User_LDAP\\LDAPProviderFactory',
      'memcache.local' => '\\OC\\Memcache\\APCu',
      'filelocking.enabled' => true,
      'memcache.locking' => '\\OC\\Memcache\\Redis',
      'redis' =>
      array (
        'host' => 'localhost',
        'port' => 6379,
        'timeout' => 1.5,
      ),
      'mail_from_address' => 'noreply',
      'mail_smtpmode' => 'smtp',
      'mail_sendmailmode' => 'smtp',
      'mail_domain' => 'tugraz.at',
      'mail_smtphost' => '127.0.0.1',
      'mail_smtpport' => '25',
    );
    

The output of your Apache/nginx/system log in `/var/log/____`:
129.27.3.13 - - [27/Nov/2019:08:44:46 +0100] "GET /nextcloud/index.php/apps/logreader poll?lastReqId=AggvOLBLLRJy4RYEtKt3 HTTP/1.1" 200 2367 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:70.0) Gecko/20100101 Firefox/70.0"
129.27.3.13 - - [27/Nov/2019:08:44:46 +0100] "GET /nextcloud/index.php/apps/logreader/poll?lastReqId=A4qnlSwTFVRi9rTJCZfn HTTP/1.1" 200 3979 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:70.0) Gecko/20100101 Firefox/70.0"
129.27.3.13 - - [27/Nov/2019:08:45:15 +0100] "GET /nextcloud/ocs/v2.php/apps/notifications/api/v2/notifications HTTP/1.1" 200 751 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:70.0) Gecko/20100101 Firefox/70.0"
129.27.3.13 - - [27/Nov/2019:08:45:02 +0100] "GET /nextcloud/index.php/apps/logreader/poll?lastReqId=Mo2TWiy7NnOTAHj4MgYv HTTP/1.1" 200 943 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:70.0) Gecko/20100101 Firefox/70.0"
129.27.3.13 - - [27/Nov/2019:08:45:22 +0100] "GET /nextcloud/index.php/apps/logreader/poll?lastReqId=Mo2TWiy7NnOTAHj4MgYv HTTP/1.1" 200 943 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:70.0) Gecko/20100101 Firefox/70.0"
129.27.3.13 - - [27/Nov/2019:08:45:45 +0100] "GET /nextcloud/ocs/v2.php/apps/notifications/api/v2/notifications HTTP/1.1" 200 751 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:70.0) Gecko/20100101 Firefox/70.0"
129.27.3.13 - - [27/Nov/2019:08:45:42 +0100] "GET /nextcloud/index.php/apps/logreader/poll?lastReqId=Mo2TWiy7NnOTAHj4MgYv HTTP/1.1" 200 943 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:70.0) Gecko/20100101 Firefox/70.0"
129.27.3.13 - - [27/Nov/2019:08:46:15 +0100] "GET /nextcloud/ocs/v2.php/apps/notifications/api/v2/notifications HTTP/1.1" 200 751 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:70.0) Gecko/20100101 Firefox/70.0"
129.27.3.13 - - [27/Nov/2019:08:46:02 +0100] "GET /nextcloud/index.php/apps/logreader/poll?lastReqId=Mo2TWiy7NnOTAHj4MgYv HTTP/1.1" 200 943 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:70.0) Gecko/20100101 Firefox/70.0"
129.27.3.13 - - [27/Nov/2019:08:46:22 +0100] "GET /nextcloud/index.php/apps/logreader/poll?lastReqId=Mo2TWiy7NnOTAHj4MgYv HTTP/1.1" 200 943 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:70.0) Gecko/20100101 Firefox/70.0"
129.27.3.13 - - [27/Nov/2019:08:46:45 +0100] "GET /nextcloud/ocs/v2.php/apps/notifications/api/v2/notifications HTTP/1.1" 200 751 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:70.0) Gecko/20100101 Firefox/70.0"
129.27.3.13 - - [27/Nov/2019:08:46:42 +0100] "GET /nextcloud/index.php/apps/logreader/poll?lastReqId=Mo2TWiy7NnOTAHj4MgYv HTTP/1.1" 200 943 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:70.0) Gecko/20100101 Firefox/70.0"
129.27.3.13 - - [27/Nov/2019:08:47:15 +0100] "GET /nextcloud/ocs/v2.php/apps/notifications/api/v2/notifications HTTP/1.1" 200 751 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:70.0) Gecko/20100101 Firefox/70.0"
129.27.3.13 - - [27/Nov/2019:08:47:03 +0100] "GET /nextcloud/index.php/apps/logreader/poll?lastReqId=Mo2TWiy7NnOTAHj4MgYv HTTP/1.1" 200 943 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:70.0) Gecko/20100101 Firefox/70.0"

additional info: installed packages,

ii  libsmbclient:amd64              2:4.9.5+dfsg-5+deb10u1       amd64        shared library forcommunication with SMB/CIFS servers
ii  php-smbclient                   0.9.0-3+b1                   amd64        PHP wrapper for libsmbclient
ii  smbclient                       2:4.9.5+dfsg-5+deb10u1       amd64        command-line SMB/CIFS clients for Unix

mount via smbclient works:

root@netbeans:/var/log/apache2# smbclient -U nc //fxxxx.tugraz.at/ceph
Enter WORKGROUP\nc's password:
Try "help" to get a list of possible commands.  
smb: \> ls
  .                                   D        0  Tue Aug 13 08:45:22 2019
  ..                                  D        0  Fri Jan 18 12:45:49 2019
  data                                D        0  Tue Nov 19 09:35:50 2019
  tmp                                 D        0  Mon Feb 18 11:05:50 2019

            10737418240 blocks of size 1024. 8608311524 blocks available
smb: \>

now on the very same machine, i have a 15.0.13 install, different install dir, different database, different data dir, but (since it is the same machine, same php-, apache-, whatever version of course, where it works as expected:

and a 17.0.01 install, where smb mounts do NOT work as well, same specs of course as well.

i would be very grateful for any pointers, since this has baffled me ( and we have a installation which started with owncloud 5 to 9, then nextcloud 9 to 16, continuous upgrades, our main system, where after the update existing mounts can be used, but no new ones created )

if anymore info is needed, please let me know.

thanks alot, that did actually help, funnily, i did not think to search in german.

solution found, thanks alot