Fixer should readily mix without heating the water to 55 C. The high temp could be the problem but if the Kodak directions calls for 55 C i guess it should be fine.
You can filter your fixer and try it on a test roll of film or just a part of the roll and see if it clear.

Is it possible to buy liquid concentrate fixer in your area? if you are having problems with powder it would be a lot easier to use a liquid concentrate.