It likely didn't cool fast enough and micro-organisms did their job.
I had that happen just once. I had to clean my pans in mid boil, I shut down, drained the syrup pan into my draw off tank and proceeded to clean the syrup pan. I paid no attention to what concentrate was in the pre-heater. As I later learned, the pre-heater became a perfect breeding ground for micro-organisms and then as I resumed boiling they got worse and about 30-45 minutes later the entire syrup pan got very ropy. It took a lot of work and time to get things flowing properly again.
I'm not qualified to show this as a study, but I did learn what not to do.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.