Man that cable is toast!!!
POR-15 is great stuff when used correctly. Once that can is opened it is subject to going bad, but generally from moisture contamination that causes it to cure, skinning over on the top, etc. Trapping a piece of plastic wrap between the lid and can when closing it up helps a lot, as well as not painting directly from the can, you have to pour the stuff into a container to paint from, and don't put any excess back into the can.
I know you probably read the instructions and were careful to remove all the loose crud from the floor before painting it, but had it come that way from previous owner, I'd suspect bad prep.
Richard