[sync] I am unable to setup SyncML, "Broken wbxml?"

[Rostislav Medvěd]

Hi, this is my first ever mailing-list experience, so please excuse any mistakes I might make.

Back to the main subject of this message...
I am trying to setup SyncML to synchronize my Horde Groupware server and my Sony Ericsson W580i. I think I set up everything correctly, but I'm not sure.
My server runs on Arch Linux, horde version 5.2.22 (installed into a separate PEAR). PHP version 7.2.12, apache version 2.4.41-1.
I had to create the /tmp/sync directory and make it owned by http, but even after that it is still empty. PHP doesn't seem to log much except this (this is printed to any active console every time I try to start a synchronization on the phone):
> Broadcast message from systemd-journald at pithree1 (Sun 2019-11-24 14:22:56 CET):
> HORDE[117899]: Invalid offset 58 value encountered around position 1065. Broken wbxml? [pid 117899 on line 240 of "/srv/http/horde/pear/php/Horde/Xml/Wbxml/Decoder.php"]

I also found this in systemctl status httpd:
> [horde] PHP ERROR: Declaration of Horde_Form_Type_country::init($prompt = NULL) should be compatible with Horde_Form_Type_enum::init($values, $prompt = NULL) [pid 114661 on line 0 of "/srv/http/horde/pear/php/Horde/Form/Type.php"]
> [horde] PHP ERROR: Declaration of Horde_Form_Type_country::init($prompt = NULL) should be compatible with Horde_Form_Type_enum::init($values, $prompt = NULL) [pid 114056 on line 0 of "/srv/http/horde/pear/php/Horde/Form/Type.php"]
> PHP ERROR: session_destroy(): Trying to destroy uninitialized session [pid 117899 on line 305 of "/srv/http/horde/pear/php/Horde/Session.php"]
> PHP ERROR: session_destroy(): Trying to destroy uninitialized session [pid 117899 on line 305 of "/srv/http/horde/pear/php/Horde/Session.php"]
> PHP ERROR: session_id(): Cannot change session id when session is active [pid 117899 on line 529 of "/srv/http/horde/pear/php/Horde/SyncMl/Backend/Horde.php"]
> PHP ERROR: session_destroy(): Trying to destroy uninitialized session [pid 117899 on line 305 of "/srv/http/horde/pear/php/Horde/Session.php"]
> Invalid offset 58 value encountered around position 1065. Broken wbxml? [pid 117899 on line 240 of "/srv/http/horde/pear/php/Horde/Xml/Wbxml/Decoder.php"]
The phone only says this: "Session failed. Invalid server response."

Anything I could try? Thank you for your time.