There is something you can try but you'll loose all your mails.
Go read this
https://www.incredissimo.com/forums/sauvegarde-et-nettoyage-du-contenu-d-im-french-and-english-t41418.htmlthis is a first thing to do and you won't loose anything. But I'm not sure it will help.
What you can try is that in your message store, you'll find all your files. You empty each of them (CtrlA, then accept), it will empty your files. Try each of them, there is surely one message that blocks everything. But of course, that way, you'll loose your mail, unless you can save them on your FAI. Be sure before doing it that Incredimail is shut down