Cannot use HTML Code in event description anymore since update

Since the latest update I cannot use HTML tags in an event description anymore.
I.e. when I try to make some text in the event description bold it and click apply it just reverts back to normal text. I can’t add any HTML code to an event description at all, as it seems. As soon as I click apply every tag is removed and all is reverted to plain text. That applies for images, lists etc.

What could cause this problem, and how can I fix it?

Best regards, Wiggy.