Rafael, I had the same problem, what you can do is delete that url, you can go directly to the user portal, then see it personal files and delete the URL in question, in this way you will be able to have your dotLRN installation working well again for that class. This thread seems to have the answer
https://openacs.org/bboard/q-and-a-fetch-msg.tcl?msg_id=0005ja&topic_id=15&topic=dotLRN%20Development