The brownish, muddy whites with red and gray tint as you describe is EXACTLY what I got with the defective Tetenal kit. The blix sold as one concentrate bottle simply DOES NOT work unless it is very fresh. As a "first aid", I found that reducing blix time, from 60 seconds to 40 seconds, helped to reduce the stain, but it can lead to insufficient blixing. Oh, the blixing was insufficient anyhow.

Leaked Rollei bottles? HOW SURPRISING!

It is possible that the stop bath is also part of the reason, but I would vote defective blix because of my experience that sounds exactly the same. Do you have any crud in blix, too?

Anyhow, add the acetic acid (2%) stop (30 sec?) and a quick rinse after it (15 sec?) first, just to be sure.