Problem solved!
Just wanted to divulge I figured this one out. Problem solved. It had to do with the container in which the fixer was stored. It was an inherited DataTainer, which was insufficiently washed. I used some fixer mixed at the same time as the one I used, and no more problem.
I'm relieved the problem was that simple. It looked really bad when I poured some hot water into the container, shook it up, and poured the contents out. Full of debris and deposits. It took several turns with a mild detergent and a round dish brush to get it all out, but it's now a usable container.
The negs were fortunately not important, it was a test roll, so no big deal.
Thanks everybody for your contributions,
- Thom