I’m really sorry, I was stressed when I wrote the above post – I meant “event series”, not “contact series” (can I still edit my post above?). Hence your comment on contacts storage doesn’t apply. Is there something similar I can do for calendar events?
I’ve been in touch with Huawei in the meantime, and they told me their phone didn’t fully support the CalDAV protocol, by the way.
Regarding logs: I can’t provide web server logs as this is my employer’s Zimbra server. I can, however, provide an anonymised extract of my DAVdroid logs, if that helps? If yes, I’ll do that asap.
Another data point: The problem does not occur with an event series created on Nextcloud. The issue only seems to apply to the specific set-up Zimbra + invite to an event series + Huawei phone.