Please help us help you and fill where relevant:
Your LimeSurvey version: LimeSurvey Community Edition Version 6.5.3+240415
Own server or LimeSurvey hosting: Own server
Survey theme/template: fruity_twentythree / sea_green
==================
I have three survey structure files that were exported from an old LimeSurvey installation. They have names like 'limesurvey_survey_<id>.lss'.

I installed a fresh copy of 6.5.2 and attempted to import the .lss files.

When I did so, I received the following error:

Import survey data
Error
Unable to import survey.
You are not allowed to use this group
[Main admin screen]

The same error appeared for all three .lss files.

I had a look at the contents of the .lss files. They look like valid XML.

Is there any way I can find out what's wrong with the .lss files?

I also did a ComfortUpdate from 6.5.2 to 6.5.3, but it didn't help with the import problem.

Thank you!
 

Old LSS files should be no problem to import. The other way round there could be issues as the structure of LSS files have changed recently. But LSS from older version should be possible to import, just not the other way round.

You are not allowed to use this group

Not sure what this is about. I have never seen this. Any chance of getting a screenshot? Is there any additional information?

Any chance you can provide one of these old surveys as LSS file here so we can test?