I have found the problem, which was in the database upgrade logic. It had only appeared when there was more than one upgrade step = when the database version has been upgraded from 2 to 4 = when DAVdroid is upgraded directly from <1.9 to ≥ 1.9.4. This also explains why most people (who updated to versions between 1.9 and 1.9.4) didn’t experience the problem.
Should be fixed with e09714af. We will release a new version soon. Please note that if you upgrade from v <1.9 to 1.9.4 ≤ v < 1.9.10 directly, the upgrade will still fail. In this case, you have to uninstall DAVdroid and install it again (or to delete all DAVdroid accounts, clear app storage and add the accounts again).
Upgrading from any version, including <1.9, to 1.9.10 should work without problems.
Thanks for reporting.
