[Metalab] Cryptography Challenge

Hello Metalab, my father just posted the following on Facebook: This is a question. The code was invented by me and a friend back in the 80s: *aCBLFDZHDFLMIY/PKJJTIFBPCZASGNYIGIWWTNJKXZAOKMTTUGQSGXENU* If someone should be able to read this, send me the correct answer. I can't imagine this code is breakable... The result should be a German text, although he said he doesn't think this information would be helpful. Who wants to try and break it? Have fun! Ben