I pulled a print out of the tank. The background is cyan. The rest is perfect. Not close. Perfect. I'm the only one who would notice the off background. Should I accept the off background?
Could the old bleach be an issue? The developer and stop are fresh. If it was a chemical issue I'd expect a bigger screw up then just the wall. No?
For what it's worth IIRC the film is Agfa Optima on Kodak Supra.
There should be no problem caused by re=bleaching. It is impossible to over bleach the film since the purpose is to remove 100% of the silver. Of course you must use care to avoid physical damage.
What color was the background? I ran into this once and it turned out to be fluorescent brighteners in the wall paint. Evidentally, there was enough UV from the flash to cause the cyan fluorescence.
I don't know if I was doing anything wrong, but in some cases I've found that reaching a perfect colour balance on all tones was just impossible. If I would concentrate on skin tones or the grass or whatever else, I just couldn't get the background to be anything else than - guess which color? - cyan. Though the slide was just perfect.
I printed the rest of the roll. It's fine. Now the enlarger settings are a little lower on the yellow so I'm guessing the cyan is some combination of the flash and the room light.