I couldn't find the "Mountpoint will not accept this URL update" message in the Icecast sources, but I do see it in the icecast-kh sources, so I guess you're using that? One cause of it is that the metadata update is from a different IP address than the stream's source. Might that be true in your case, eg. perhaps there may be multiple routes out of your LAN to the internet?
When the problem happens, is the stream still playable?