Sync error with write-protected holiday calendar

I imported the (a) German holiday calendar with worked well, firstly. However I get a sync error telling me that em cannot upload data to this write-protected calendar:

As I do not actively upload any data I wonder why I get this message?

As soon as I remove this calendar from the sync list the error message disappears.

What do I have to change to be able to include the holiday calendar without getting sync errors?

Update 25.5.: The 2nd error message “Error in Client/Server conversion” has another reason and is not related to the Holiday calendar. What can I do to identify and solve this problem?

Thx for help,

Hello Alfons,
can you please check Help>About section and copy the exact version number of eM Cleint for me?
Does this problem happen with other Holiday calendars as well?
Can you copy the Log (Protokoll) window for me next time this error pops up? (It could contain more information about the error)