In the meantime I tried a second recovery tool and the result is identical, which means that most probably the data is gone, as all files recovered seem to be overwritten with a similar pattern. It is not '00' or 'FF', what I would expect, but a repeating string.
My guess with the encrypted disc is probably wrong as I can read from the disc, so I assume that the data (even the deleted one) is decrypted correctly.
The files were deleted by 'WinMerge'. Maybe this tool is not just marking deleted but actually overwrites the data, or then, you are right, it's because it is a SSD. After deleting I did not do anything on the disc, as it is a pure data disc. The chance of being overwritten is approx. zero. But of course, I don't know what a SSD does on its own.
I'm trying to recover some data on an external HD, deleted by windows explorer, lets see if this works...