Horde active sync stopped syncing calendars

I updated my UCS server to the current 4.4-1-6 errata241 (i was still somewhere on 4.3…)
Now i realised that all mobile devices stopped syncing the calendar via active sync.
In the apache log, i see positive “200” records, that the connection is ok.
In the horde log within the docker, i saw on message, the PDO connection could not be establised, but this was only raised once during the update, one week ago, not further errors were logged here.
No matter where i create a calendar entry (within horde accounts, or on mobile device calenders) entries are not synced.

Where could things go wrong?

active sync is enable in horde conf.d/

<?php $conf['activesync']['enabled'] = true; $conf['activesync']['emailsync'] = true; $conf['activesync']['version'] = '12.0'; $conf['activesync']['auth']['type'] = 'basic'; $conf['activesync']['autodiscovery'] = 'full'; $conf['activesync']['outlookdiscovery'] = false; $conf['activesync']['logging']['type'] = 'horde'; $conf['activesync']['logging']['path'] = '/var/log/horde/'; $conf['activesync']['logging']['type'] = 'custom'; $conf['activesync']['ping']['heartbeatmin'] = 60; $conf['activesync']['ping']['heartbeatmax'] = 2700; $conf['activesync']['ping']['heartbeatdefault'] = 480; $conf['activesync']['ping']['deviceping'] = true; $conf['activesync']['ping']['waitinterval'] = 5;

Update:
i unpaired all devices, and repaired to see if that changes something.
i watched the horde logfile in the docker container and found this:

2019-09-04T10:50:03+00:00 ERR: HORDE Returning HTTP 500 while handling Ping command. Error is: Received FALSE while handling Ping command. [pid 21693 on line 160 of “/usr/share/horde/rpc.php”]
2019-09-04T10:50:03+00:00 ERR: HORDE Error in communicating with ActiveSync server: Received FALSE while handling Ping command. [pid 21693 on line 165 of “/usr/share/php/Horde/Rpc/ActiveSync.php”]
2019-09-04T10:50:03+00:00 ERR: HORDE #0 /usr/share/horde/rpc.php(160): Horde_Rpc_ActiveSync->getResponse(NULL)
#1 {main} [pid 21693 on line 165 of “/usr/share/php/Horde/Rpc/ActiveSync.php”]
2019-09-04T10:50:03+00:00 ERR: HORDE Buffer contents: [pid 21693 on line 165 of “/usr/share/php/Horde/Rpc/ActiveSync.php”]
2019-09-04T10:50:08+00:00 ERR: HORDE Returning HTTP 500 while handling Ping command. Error is: Received FALSE while handling Ping command. [pid 21696 on line 160 of “/usr/share/horde/rpc.php”]
2019-09-04T10:50:08+00:00 ERR: HORDE Error in communicating with ActiveSync server: Received FALSE while handling Ping command. [pid 21696 on line 165 of “/usr/share/php/Horde/Rpc/ActiveSync.php”]
2019-09-04T10:50:08+00:00 ERR: HORDE #0 /usr/share/horde/rpc.php(160): Horde_Rpc_ActiveSync->getResponse(NULL)
#1 {main} [pid 21696 on line 165 of “/usr/share/php/Horde/Rpc/ActiveSync.php”]
2019-09-04T10:50:08+00:00 ERR: HORDE Buffer contents: [pid 21696 on line 165 of “/usr/share/php/Horde/Rpc/ActiveSync.php”]

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