I use the app “CardDAV-Sync free” by Marten Gajda for syncronising my calendars with my mail account at Posteo.
This night the app was updated via the Play Store to version 0.4.10 and since then I cannot access my contacts anymore - the app asks me to enter my password repeatedly, only to give me an authentification error message.
I use the same developer’s app “CalDAV-Sync”, and it still works fine.