Hello Romy,
It should work just as you’ve done, so something is wrong – either with the file or with the program. We’ve actually not had any reports like this, so we’d like to investigate this further.
Please follow the instructions here: https://forum.axcrypt.net/blog/send-complete-error-report/ .
This should give us some more information to go on, to understand just what and where it goes wrong.
You can always download the legacy AxCrypt 1.x (in this case I’d suggest the legacy AxDecrypt.exe standalone program since it can co-exist with version 2) to decrypt it, by browsing to http://www.axantum.com/LegacyStart.html and select the appropriate download there.