Yep.
It's a bugger to get at, the bike has to be mostly naked, but a fairly cheap and undramatic fix.
This is the part. I couldn't find it at Stein Dinse, although I thought I bought mine there last year.
https://wendelmotorraeder.de/oelschlauch-entlueftung-dick_gu30157400-p-1009234.html?ref=expl
When you do your "tank-off maintenance", it's worth considering changing that out on spec. As I said, it is a bugger to get to. The tube gets old and hard, and splits eventually. That looks like your pictures, or if it is the other end (as mine was) like a leaking output shaft seal on the motor. Whilst you're in there, if in any doubt at all about it's integrity, replace it, even if it is not the cause of your current leak.