We had that happen in our group, and it wound up being Adamastor causing it. We substituted a custom mob and the error stopped and we were able to play the arc.
Live arcs: 517377 and 517381
Virtue: Quickshot. Swiftwind. Aliuneidis. Gizmodeus. Dasher. Fiver. Inuit Acer. Daniel Darke. Cerebral Flame. El Halcon.
Intel Core2Duo 2.4 Ghz 4 GB RAM**NVIDIA Geforce 9600 GT set to 1280 x 1024**Windows Vista 32 bit
Yeah, the problem for me was a Coralax boss. Seems some particular enemies cause problems.
I went to try out an arc I published today, and it refuses to let me enter - clicking the portal gives a pop-up with the message "MAdoorinvalid" and opens the submit comment/rating window. Searching the forum and the internet for that phrase reveals nothing. Any idea what's causing this? It seems really odd.