Hardy, Megiddo, and even I, have had trouble replicating the errors you are reporting.
I'm betting there is an addon conflict causing prop monitoring/spawning to break the hooks used by UPS for those events.
While you and the users experiencing those errors are connected to the server, enter this command, "ulx debuginfo", at the server console and attach the file it creates here, or paste the output into the thread. Please note that it will output to a file, not to console.
When you run that command, it tells you where it created the file.