Thank you for the debug logs. However it still seems to be a problem with the server. CalDAV/CardDAV servers must send an E-Tag (to determine if resources have changed) and yours does not seem to transmit it to DAVdroid… Maybe try another server and import your resources manually?
Option in FAQ's to set synchronization interval does not exist?
The FAQs have a page describing the process for adjusting sync interval from the default of one day. When I try to do this, the option does not exist - I see only cancel or remove account. Everything else works perfectly but I’d love to get sync working every 30 minutes or so.
Settings have been introduced in DAVdroid 0.7. They are accessed either from the “DAVdroid Accounts” overview - (not inside a certain account, Android requires a seperate section for Account settings!) or via the app start screen.