This seems to be a
a) Transport issue where the function module has not been moved to the system. If you suspect this to transport issue , then check the other systems in the landscape.
b) Somewhere a FM is being called , which is deleted . If you suspect this to missing FM issue , go to the call stack in st22 , check from where it is called and finally find out who has last changed the erroneous class. Usually this should also lead to the syntax error.
~Kavindra