Uh... I'd first try to investigate which script is trying to rename the library. There's no point to rename it constantly. Do you have something unusual in WBStartup drawer? A script which would try to do that? IconX is usually used to launch scripts from icons...

You should keep the dummy library renamed to rexxsyslib.library_disabled in mossys:, because then MorphOS installer knows that it shouldn't overwrite the rexxsyslib.library when upgrading the system. If you only have one file there, it will be gone in the next system upgrade and you have to remember to copy it back every time yourself.