Diafine - What is this stain?

Here's a clip of Tri-X that I developed in Diafine. I placed it inside a camera and exposed the camera to sunlight to see where I might have some light leaks. I never opened the shutter. I see that I have some light leaks around the film door, but that's not what I'm interested in now. What is the stain in the middle of the negative? My Diafine was mixed up at the end of last year, but has only developed two rolls of film -- the most recent last week and it looked good. This was developed in a Paterson tank, 3+3 at ~74F, twirl agitaiton for five seconds of each minute in A and B. Water stop, fix, rinse, as usual. (The yellow mark was made with a sharpie along the cassette.) Any ideas?

Try fixing again.

Thanks. That seems to have worked. I wonder why it didn't work the first time.


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Shot in the dark guess, but there would be more unexposed silver than usual on that kind of test film.... or possibly if the water stop was just a quick fill and dump, carryover from very alkaline part B might have inhibited fixing or made it a bit slower than usual? Glad the main mystery is solved though

That stain is caused be either:

1: exhausted fixer (remedy: mix up a new batch and re-fix

2: Hasn't been fixed for long enough.

It has nothing to do with the Diafine.