I think your problem occurs before the indexing. "Cannot update cursor" is probably a response to your "INDEX ON cldpart_num TAG cldpartnum ADDITIVE" command, which fails for some reason. I expect that "Index tag not found" is a symptom of this failure, not a cause.

As for why a cursor created with "CREATE CURSOR" can't be updated, it could be system-related: out of disk space or memory, the OS has only allotted a certain amount of space in \Windows\Temp, etc. - this could explain why it came upon you suddenly. Sometimes an OS (especially Novell) marks files or directories read-only or unsharable. I'd look for any recent maintenance done on your LAN, OS upgrades, etc.