Yeah I would agree with the other post. Try a mass compile (the link mentions LabVIEW 7.X but it is true for 6.X as well, so don't be put off). It can take a while and a full system backup isn't a bad idea either, first. Defrag your hard disc as the perfomance will be better if the hard disc is tidy and watch out for crosslinked files, hence the backup suggestion (Windows in particular).
If you have installed a later version of Labview (and accidently compiled a common routine) or moved in some VI's from another version, it can get confused. Possibly you have accidently moved in or a VI is being included via a path statement, that is causing the problem.
Of course you will have some backups of the versions that worked 😉 - So you could go back. But I found in the past it was generally in a Library routine / shared library(s), not always included in the backups held for the project (library routines being backed up with a full system backup and also availalble from source distributions). On the odd ocasion I had to sort out which library(s) needed refreshing, even from the original distribution. But hopefully the mass compile should clear it all up.