There is a agent determination rule that is not working (it is not returning any agent). Now I don't know which kind of template you have, or what kind of steps there is, but let's say that the following step was an approval step, there should be an agent rule defined for the step. This rule is not working!
Did it work before? Can you test the rule in transaction PFAC? Does it return an agent?
Regards,
Karri