When a new calendar entry is created, akonadi uses HTTP-PUT to create the entry on the server site.
Zimbra however has the limitation that the ics-files have to be named using an UUID in the common format a5792563-68fc-4387-9759-72c95c5ea164. See https://wiki.zimbra.com/wiki/CalDav_Support under "Known Limitations".
Akonadi should either use UUIDs as file names or use POST to let the server choose the name.
As Akondadi uses some different format, the put-request fails and the Resource is set to offline and not synchronized anymore.
I am under the impression that this worked recently. I'm running the stable version of kde neon, so a recent version might have been working, but I cannot guarantee that.
Of course one might consider this a bug/missing feature of Zimbra, but it is unlikely that Zimbra will change that behavior as it seems to be due to how they organize calendar entries.