Thanks for the reassurance that I'm not the only one who's done this! LOL
Actually, after much time with Epicor support, several queries going nowhere, and much time banging my head on the desk... on a whim, I decided to test the Security group, as soon as I put one of my users in a new group, he was able to proceed! I think, somehow, the security group got corrupted?? I'll try and confirm that with Epicor support later today.