Hi @Denony ,
I’ve now tried to receive the midi clock from the S6D to the KS37 but also without any luck.
So I guess either I do something wrong, or the EXT CKL mode 1 on the S6D is buggy.
Although, when I hook up other gear(sending midi clock out (via DIN)) to the Keystep 37 it works like a charm.
The tempo is correctly displayed on the ks37 and also the transport messages don’t get mixed up.
But that doesn’t help you with your problem 
I just wanted confirm that part.
Have a great evening/day and have fun playing music.