Peter,
I also have Win10 and also had got the 'trial period expired' today and entered my serial number again. I then imported a revized file and opened the program to ger the same message of can't access..... (I also have GData). I've had TMG9 on Win 10 since last September and other than putting in the serial number periodically it has been okay until now.
However, you may have done this as a workaround, I hit 'Ignore' and the data can be accessed [so far], but it would be nice to not have the error in the first place. You are not alone, but, obviously, you use the software more than I as I use it to make reports.