Thank you for your replies. That is pretty much what my shop warned me about. What sealants have you tried (I guess there are three or four manufacturers that offer them) or did you have bad experiences with all of them?
I was planning to get the evaporator replaced anyway, just not right now. The 500E is my daily driver and this is the first time it will be down for more than one day. It has been flawless in performance and reliability so far (126000 miles). I hope this is not a sign of things to come...