Outlook Express 5.5 Errors

Running Outlook Express 5.5 on Win 98 SE. All of a sudden we regularly get the following error, usually after just opening outlook or after just switching identities. We have several identities:
This program has performed an illegal operation and will be shutdown. MSIMN caused an invalid pagefault in module URLMON.DLL at address…
I’m not positive that it’s always the same DLL that shows up. A different dll shows in in Microsoft search, pointing to a .dbx file with read-only attribures. I checked and none of mine are.