Nextcloud funktioniert nicht mehr nach Upgrade zu Debian 10

Moin,

ich habe eine Nextcloud 15.0.10 Installation auf meinem Debian Server.

Mit apt-get update, apt-get upgrade und apt-get upgrade habe ich auf Debian 10 geupdated. Lief auch alles gut.

Auf alle Seiten meines auf apache2 basierenden Webservers komme ich rauf. Lediglich beim Aufrufen der Nextcloudseite kommt folgende Fehlermeldung:

"Internal Server Error

The server encountered an internal error and was unable to complete your request.
Please contact the server administrator if this error reappears multiple times, please include   the technical details below in your report.
More details can be found in the server log."

ich habe mir die nextcloud.log angeschaut, werde daraus aber nicht schlau:

{"reqId":"yLCKjvMxDLMGV5eSGJUO","level":3,"time":"2019-07-   28T15:27:49+00:00","remoteAddr":"91.106.187.199","user":"--","app":"remote","method":"GET","url":"\/nextcloud\/sta$
{"reqId":"5jPebuCSPiTy95oaiWyq","level":3,"time":"2019-07-28T15:27:51+00:00","remoteAddr":"91.106.187.199","user":"--","app":"PHP","method":"GET","url":"\/nextcloud\/ocs\/v$
{"reqId":"t28j09H5e3CpmD1O0llx","level":3,"time":"2019-07-28T15:28:21+00:00","remoteAddr":"91.106.187.199","user":"--","app":"remote","method":"GET","url":"\/nextcloud\/sta$
{"reqId":"xuFLolL1wfWlCVglYH81","level":3,"time":"2019-07-28T15:28:23+00:00","remoteAddr":"91.106.187.199","user":"--","app":"PHP","method":"GET","url":"\/nextcloud\/ocs\/v$
{"reqId":"IsBOy3YM9rAjWqjCTRIR","level":3,"time":"2019-07-28T15:28:51+00:00","remoteAddr":"91.106.187.199","user":"--","app":"remote","method":"OPTIONS","url":"\/nextcloud\$
{"reqId":"IsBOy3YM9rAjWqjCTRIR","level":3,"time":"2019-07-28T15:28:51+00:00","remoteAddr":"91.106.187.199","user":"--","app":"core","method":"OPTIONS","url":"\/nextcloud\/r$
{"reqId":"IsBOy3YM9rAjWqjCTRIR","level":3,"time":"2019-07-28T15:28:51+00:00","remoteAddr":"91.106.187.199","user":"--","app":"core","method":"OPTIONS","url":"\/nextcloud\/r$
{"reqId":"kPdagQxtlppZ6pbNJ8Vp","level":3,"time":"2019-07-28T15:28:53+00:00","remoteAddr":"91.106.187.199","user":"--","app":"remote","method":"GET","url":"\/nextcloud\/sta$
{"reqId":"xG0EiSlbx3kEaq0CfQJT","level":3,"time":"2019-07-28T15:28:55+00:00","remoteAddr":"91.106.187.199","user":"--","app":"PHP","method":"GET","url":"\/nextcloud\/ocs\/v$

Was ich noch gemacht habe: ich musste php-mysql neu installieren, weil das Paket nach dem Upgrade fehlte.

Hier noch zur Info meine config.php:

<?php
$CONFIG = array (
  'instanceid' => '***',
  'passwordsalt' => '***',
  'secret' => '***',
  'trusted_domains' => 
  array (
0 => '192.168.178.32',
1 => '***.org',
  ),
  'datadirectory' => '/Nextcloud',
  'htaccess.RewriteBase' => '/nextcloud',
  'overwrite.cli.url' => 'https://***.org/nextcloud',
  'dbtype' => 'mysql',
  'version' => '15.0.10.0',
  'dbname' => '***',
  'dbhost' => 'localhost',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'dbuser' => '***',
  'dbpassword' => '***',
  'memcache.local' => '\\OC\\Memcache\\APCu',
  'memcache.locking' => '\\OC\\Memcache\\Redis',
  'filelocking.enabled' => 'true',
  'redis' => 
  array (
    'host' => 'localhost',
    'port' => 6379,
    'timeout' => 0.0,
  ),
  'installed' => true,
  'maintenance' => false,
  'loglevel' => 2,
  'logfile' => '/var/log/nextcloud/nextcloud.log',
  'mail_smtpmode' => 'smtp',
  'mail_smtpauthtype' => 'LOGIN',
  'mail_smtpauth' => 1,
  'mail_smtpsecure' => 'tls',
  'mail_from_address' => '***',
  'mail_domain' => 'web.de',
  'mail_smtphost' => 'smtp.web.de',
  'mail_smtpport' => '587',
  'mail_smtpname' => '***',
  'mail_smtppassword' => '***',
  'theme' => '',
  'updater.secret' => '***',
  'mysql.utf8mb4' => true,
  'updater.release.channel' => 'stable',
);

Geändert habe ich an der Config nichts händisch nach dem Upgrade zu Deb10.

Was steht in der error.log?