Oh my god!
I just registered here to say: Thank you so much! I was struggeling with this problem for too long. I contacted bitfire in the past but they kept telling me that this would be an owncloud problem. They told me to wait until Owncloud 9 and all the problems would be gone because they completely reimplemented the calendar/contact stack for OC 9,
However, after upgrading to OC 9 the problems remained. Just now I realized that it is an apache issue. Why the frack did they turn off ETags by default in 2.4?
In this thread they say that they didn’t change anything about etags in davdroid, but this is wrong. The support told me that they actually changed the behaviour after version 0.8.something. So I kept using this version before I read the solution on this thread.
Again, thank you!