@6042114fa0 I see, sorry, I haven’t read the whole thread again. There is no Huawei image in the Android SDK.
Unfortunately I don’t have any further hints. I guess it must be something specific to your device. If there’s something certain I can do for you, just let me know.
Finally figured out the reason for this. In Zimbra, there is a recurrence rule “Every weekday”. This causes the error. Setting the event to the (equivalent) rule “every week” (with only “Monday”, “Tuesday”, “Wednesday”, “Thursday” and “Friday” enabled) does not cause the error.
It apparently doesn’t like this bit:
RRULE:FREQ=DAILY;BYDAY=MO,TU,WE,TH,FR,
…whereas this one is accepted:
RRULE:FREQ=WEEKLY;INTERVAL=1;BYDAY=MO,TU,WE,TH,FR
Can we do anything about this?
@6042114fa0 RRULE expansion is done by Android (calendar storage) and not by DAVdroid. DAVdroid only stores the string into the RRULE field of the event.
FREQ=DAILY has worked in my local test, so it must be specific to your Android version/firmware. Can you update to a newer Android version?
I am of course using the newest Android available for my Huawei P20 Pro.
It must be due to DAILY being combined with a set of days.
@6042114fa0 Do you think there is anything I can do in this case?
Not sure whether you’re in regular touch with the support teams of device manufacturers? Probably more than myself.
@6042114fa0 We file some reports from time to time (if we can find a way to do so), but usually we don’t get an answer nor are the requests taken seriously. Seems that as long as Google calendar sync works, there’s nothing to do 
I can try with an older image, 4.4 from the emulator and have a look there. Maybe it was a vendor-independent Android problem and has already be fixed (but maybe not in your firmware yet).
I have tried again and couldn’t reproduce the problem with plain Android (Android image from SDK), neither with 4.4 nor with 9.0.
However, I have seen that we have received one or two reports about this problem earlier. Seems that some special devices don’t like RRULEs with DAILY…
Like here in the forum? Didn’t find any.
@6042114fa0 I have found emails where this was reported … but very rarely.