if you consider THIS a "novella", then
my posts must be considered ENCYCLOPEDIAS !!!

Your thought processes seem to be going in the
right track for debugging.
You have to remove the "variables"... the things
that make one case different from another.

Since you're importing and exporting the userform,
and one works while the other does not, then
the difference (or "variable") is in the userform.

The difficult part here is that Microsoft MAY be
giving you a "default" error message.
The problem could actually be in the code itself.
Like, did your UPDATE include some functionality that
requires excel to have another add-in that is not
included on THIS user's machine? In which case,
the actual error is that it couldn't resolve something,
but didn't know how to say it, so it just says
"file not found"... it's not the USERFORM that's not
found, but the ADD-IN (or resource, or...)

Something to try...
Import the userform into the workbook instead of Personal.xls.
See if it works from there...

==========================================

Now... a different, yet troubling side-comment.
If you want to try something a little different..
You indicated that in order to "distribute" your changes, you
have to export and import the userform into the Personal.xls file.

Is this because the userform isn't tied to a specific "application"/excel file?
In other words, the userform isn't using data from a file that has to be open
to work?

I have an excel file that has 10 userforms and 15 modules.
The file is used to retrieve and rebuild documents from an Oracle database.
There's over 30,000 lines of code. I'm continuously "improving" the code.
Ok... I'm fixing it... When I'm ready to "publish" the changes, I export the
modules and forms to a shared folder. When the user next opens the application,
I use the Workbook_Open event to load all forms and modules from the shared
folder,
so they always have the most recent version.
When the document is "published", I didn't want to include the forms and
modules,
so I use the BeforeSave event to delete all modules and userforms.

so you see, I never have to worry about visiting 180 workstations and making
sure
they all have the most recent "update"...

The ONLY concern I currently have is that I've hard-coded a password into the
workbook_open event so that I restrict access to the VBA code.
If the user is VERY quick, he can open the workbook and hit the "escape" key
and interrupt the imports before they finish and reset the protection.
If they do that, then they COULD read the VBA code and get my password.
I believe it's possible to disable the 'escape' key functionality, but I haven't
researched it.

If you want some sample code for doing something like this, let me know...

now, you still think you're a "novella" writer??? ;-)