We have been struggling awhile with the "Invalid user ID or password." issue with Epicor 905.602A.
Every one to two weeks, Epicor refuses to let us log in. The log files show nothing, and Management Console shows the app server running fine. However, if you stop and restart the app server, the problem goes away. Connecting using WCF instead of the Epicor Client returns the same error message.
On our last support call, Epicor Technical support has a possible solution. They believe the issue is with OpenEdge, not Epicor itself. We are running 64-bit OpenEdge on Windows.
To attempt to fix this issue, we are applying HotFix 21 (rl102ASP0321hf-64.EXE), available for download from the Epicor support site (you need an account to login). Because the problem occurs intermittently, I won't know for sure if the fix will work, but it is nice to have a course of action. Here's hoping.