Same Problem here.
Since the latest update from Enpass 5 to Enpass 6 Beta on my Linux System and my Android Phone the synchronization with my Nextcloud Server doesn't work anymore. I can connect to my account, but it won't sync the data.
The error code on my Phone is 915993
The error code on PC is 907016
The previous separate Beta (can't remember the version no.) from the Beta Channel worked fine on all my Devices.
It's definitely the latest Beta causing me problems.