Ok, thanks for that detailed explanation, that makes perfect sense now. It’s not so much that there was an error in the registercustomfunction statement in particular, but simply that there was any runtime error in the .InitializeFunctions procedure. Since this procedure is run when Panorama is starting up, the error prevents Panorama from completing the startup process, including logging in, which is one of the last things Panorama does when starting up.
I’ve filed a bug report, and also fixed the problem.