Maybe the answer you don't want, but you most likely need to modify the code to work in both environments or update all pc's to one or the other.  I've had to do this several times.

Thanks John

I've tested a bit more and found a termporary solution.
As I mentioned the app has front and back-end.
When I use the accdb file I can open it on any of the computers above with no problem.
However when I create the accde file it refuses to work.

In order for my customer to work with the software anyway, I turned off the ribbon and the
navigation pane. That way it works like the accde file.  It's maybe a bad solution, but it is one
for the time being.

By the way. I can open the back-end accdb file with Access, however it doesn't work when
I open the accdb file directly.

I guess I'll have to do some more research. Maybe Rick has a solution for this.

Thanks for your help and take care
           Mike

PS: At least we have nice weather now in Sweden where I live, so I can't complain.

OK following along and trying to think outside the box. I'm interested in the issues because I'm about to travel down this road myself.

What if you create a new blank 64bit db.  Then import all of the tables, forms, etc into the new one from the 32 bit.
Theoretically the conversion should be seamless. But we don't really know what MS team did in the conversion process.

Then create the ACCDE from the new 64bit db.

Hi Scott
Yesterday I took the 32-bit database ( data only ) and opened it in a 64-bit access. For me there is no way to
control it whether it is done or not.
Then I took the front-end into the 64-bit access and linked the 'changed' database. It worked so far in the
accdb mode.  
Next I created the accde file and got a message like:  The file as been created on a 64-bit version, use it on a 64-bit access
version'  and that was it. Nothing except that message showed up.
Normally when one double-clicks a file name ( database accdb ) the database is shown.
Not here though - just an empty screen.

I am going to go through the whole routine again and make a few hard-copies.
I never expected it to be so troublesome to upgrade the system to 64-bit.

Take care and I'll keep you informed.
Have a nice weekend.

Hi Scott
sorry it took a bit longer than expected because my grandchildren were quite sick and I took care of them.

However during that time I tried a few times but it just didn't work.

The reason being:
Before I had front-end and back-end on dropbox for reasons of automatic and secure saving .
Another reason was that I could directly interact with the databases of my customers.
It certainly worked for the last 12 years.

Then I did this:

1. De-installetion of all MS Office applications, incl. Runtime.
2. Installation of MS Office Pro Plus 2021 on .
3. Place the front-end ( program ) and back-end ( database ) directly on the computer ( C:  )
4. Start the application with MS 2021
5. It worked.  ( As I mentioned in the beginning I added PTRSAFE in all relevant modules )

Now I had the problem with the dropbox, which I needed to be able to check when my customers had a problem.

Since it worked when I got the database on C:  it was logical that the problem arose with Dropbox.
Then I realized, that dropbox accesses the files continuously because of synchronisation. Therefore the error message 'database in use'.

I then placed the back-end ( database ) on dropbox again, stopped synchronisation, created an accde-file and - low and behold - it worked.
After that I started synchronisation again and I now am happy.

The problem with the error messages is always the same. The error doesn't make clear what is wrong.

Hopefully this helped.

Take care
           Mike

Mike you figured out the issue yourself.
Access does not work on file sync programs correctly with live data.  The same applies to Google, OneDrive, etc.
It is great as a folder to copy a back up to but not to share files.

Check out the File Sharing video.