This is a server error: the server doesn’t send ETags on GET responses, which are required for synchronization.
Please take logs and send them to your server admin/manufacturer. You may also post them here just to be sure.
When I change phone locale to Persian, I get this error:
Unexpected char 0x6f2 at 21 in User-Agent
value: DAVdroid/1.11.2-ose (۲۰۱۸/۰۵/۰۵; dav4android; okhttp/3.10.0) Android/8.1.0
Here is the debug info: 0_1527489814700_debug.txt
we’re sending you an apk via email. This should have been fixed in the latest version. Does it solve your problem? It will take some time until this is coming to f-droid I guess…