This is really weird. I had the exact same thing yesterday. I boil on a 2x5 flat pan, continuous flow setup.

My sap was 3%. I started the day with sweet pans, and was ready for a draw pretty quick. I drew off until the temp. dropped a degree, then closed the valve. After that I fired, and kept on as usual. Then the temp dropped to 215, lots of foam, and stayed there for the rest of the night (3 or 4 hours). The drawoff compartment temp. was 215, the next one was 214, and the 3rd was at 213.5, so I had a gradient but it just wouldn't finish. The way it looked and acted in the pan I could swear it was syrup, but the hydrometer would just barely stay upright in the cup.

When I shut down, I pulled 2 separate pails, about a gal. each to reset the gradient tonight. Hopefully after shutting down with flooded pans, I can re-establish the gradient tonight.