So somehow the “:” has probably slipped into the URI which - of course - would make it illegal.
Actually, ":"s are legal in URL path names, which has already caused many problems because servers actively use it. DAVdroid should be able to handle it correctly. But maybe it would have to be encoded in the .ics or something like that.
However, I’m glad that it works for you and that the problem is now gone. 🙂