iCal / CalDAV: Unable to invite others - OS X Mavericks / Server

Hello,

we recently upgraded our OSX Server from Mountain Lion to Mavericks including the Server App. We ran into migration problems with our iCal Database.

Once the DB got fixed, thanks to community help, everything seemed to work. Now after two months one person reportet that she is unable to invite a specific subset of people from the company. I checked her account and can confirm the issue.

In the availability pane, the people to be invited are greyed out and not available all the time. Interestingly, when I do a calculation of the next availability, the Server concludes April 1st as the first possible date. I really hope that's not a aprils fools joke... ;-)

Any idea how to solve the issue?

Thanks!