Might be a redundant question, but how long did you keep it in the hot water and was the water hot, or just warm?
I had those same old Ursa bottles completely solidify and was able to warm them back to liquid. I put the bottle in a deep plate, that allows the water level to go up to the neck of the bottle. Boil water and fill the plate, let the bottle sit there for 15-20 minutes, check and shake. If still not liquid, replace the water with new boiling water and let the bottle sit for another 15-20 mins and see again.