Hi Igor,
Many thanks for your prompt response to my problem.
For the record I only have one MacBook Pro with the MacOS Big Sur update.
I followed your procedure and all is now working perfectly and I managed to find the previous emails with the License Keys and reinstated those for each PTE version.
I still don't quite understand how PTE AV Studio 11 got messed up, but life is too short to worry about that now that you have it fixed.
Huge thanks for solving the issue and for your great software product.
Warmest regards,
Denis S