Ah OK apologies Tamhas, this is obviously part of my problem I didn't know or understand that.. we don't touch _file either way._File is not a VST. It is schema.
Wild guess: it's possible (not at all likely, but possible) that the application does something bad like storing template record recids and updating them directly. This wouldn't involve _File reads at run time and could result in the error you see.we don't touch _file either way.
Can you reproduce this error in a test environment? Do you have source code and development tools? If yes to both then you (or someone) can step through the code in a debugger and find the offending program. Based on what you find you can address it.
If not, add -clientlog and -debugalert to the client startup parameters so you will at least have a full call stack when you hit the error. Send the client log to the developer. That should point them in the right direction.