2.599 > 2.609 update fails (integrity check)

I am running 2.599 or Rpi3 with Nanosound Plugin (Nanosound DAC)
I am trying to upgrade and I get this

System Software Integrity check failed, manually edited files were detected. System Software Update is not possible under those circumstances. Please execute a Factory Reset or Delete User Data, then retry.

I do not recall editing any files…how can I check?

Me too.

Same with next version.
Clean user data did not help so I had to download the newest image and install from scratch…
Something is wrong with the update function.