Thanks for your answer! I solved the problem using the debug mode. I did not know that it existed.
When looking at the log I realized that I have got a bad authentication (error 403). Reason is that when adding the calendar to rainlendar, somehow a wrong password was stored. I realized this when looking closely at the settings and I saw that there were about 20 stars (*) in the password field. My password is only 9 characters long. So I replaced the 20 stars with my password and it worked.
I am not sure if this is a bug or something. If you are interested in the log file anyways, I can send it to you.