Hello Stefan,
Can you please,
1.Share manual PuTTY log of commands being executed to take backup.(Should be from first key stroke to connect the device).
2.Share me the debug log of the device by run the activity without the variations. (Just disable the newly added prelogin keys/messages and save).
Also please add as attachment of those file instead of copy paste in the reply box. Attach menu can be found under 'Use advanced editor' option(this can be found on right top of reply box you are typing).
Thanks,
Nanda