I can not help with the issue so here is a translation for
to help English speakers.
Synchronization of folders Birthdays of contacts
The upload of the item “Dennis Matsiyevych (1988)” failed
You can delete the item locally or ignore the error. If you decide to
Ignoring errors now, an upload of the element will be attempted during the next synchronization.
The server responded with the error: Not Found.
The destination folder was cloud.vunlox.com/Geburtstage of contacts.
Action for all problems during this synch. Apply - Delete
Well, it is a misleading error. First the server says the folder can’t be found, then it says you can’t save in a folder it found. So either the folder doesn’t exist, or you can’t save to a named folder. It can’t be both. Therefor, server error.
(At least that is what the error looks like as my German is not that good)
But as far as I know you shouldn’t be able to save in that folder anyway. Did you try to save to that Birthday calendar folder in other applications?
It probably means that the reminder is already scheduled for upload.
After disabling the reminders for that folder, right-click on the folder again and choose Properties > Repair > Repair. This will delete the cache and resync it from the server. Don’t worry if the calendar initially appears empty, it will resync.