Contacts disappearing after one day

[sz22]

Nextcloud 15.0.4 , contacts 3.0.3 , running on apache/debian 9

After running perfectly for a long time, suddenly my complete addressbook disappeared.
After importing my contact list to the DAVx account on the android, and syncing, all contacts are back again in the WebUI. But after 24 hours, they disappear again.
I tried to turn off syncing, but get the empty address book the next day.
What can be wrong? Nothing in the log.

[skjnldsv]

Hello @sz22!
Do you have access to your webserver access log? Maybe one of your device is removing all of them?

@sz22

I tried to turn off syncing, but get the empty address book the next day.

On the WebUI ? Do you have any errors in your browser’s console?

How to access your browser console (Click to expand)

Chrome

• Press either CTRL + SHIFT + J to open the “console” tab of the Developer Tools.
• Alternative method:
  1. Press either CTRL + SHIFT + I or F12 to open the Developer Tools.
  2. Click the “console” tab.

Safari

• Press CMD + ALT + I to open the Web Inspector.
• See Chrome’s step 2. (Chrome and Safari have pretty much identical dev tools.)

IE9

1. Press F12 to open the developer tools.
2. Click the “console” tab.

Firefox

• Press CTRL + SHIFT + K to open the Web console (COMMAND + SHIFT + K on Macs).
• or, if Firebug is installed (recommended):
  1. Press F12 to open Firebug.
  2. Click on the “console” tab.

Opera

1. Press CTRL + SHIFT + I to open Dragonfly.
2. Click on the “console” tab.