Suddenly sync problems

Dear support team,

 

I have your calendar installed on my Android smartphone and on my Windows laptop. It worked really well so far, but since this week I am getting an error (see attached screenshot) that the calendar is unable to synchronize my appointments. I don't know what to do and how to get this working again, because the error message isn't informative. Do you have an idea, what the problem could be?

 

Thanks in advance!
Stefan Hackel