UDF and AMG solver crash

Josef

Please, could you help me with a boring problem. I'm using UDF for modeling chemical reaction in porous media. It's a tube reactor with catalytic layer where the reaction occurs. I have to use UDF, becasue there is no way how to do that directly in Fluent, or is there? And when I use my UDF (source terms) it always stops with this message: Error: divergence detected in AMG solver: species-1 Error Object: ()

Does anybody know what is it? Or what can cause this problem? Thank you very much, I cannot overcome this problem. Or if you have better solution how to solve this system I will appreciate your help.

Josef