divergence detected in AMG solver
 

Hi all,
I have a UDF in the model to define mass diffusion coefficients for the whole computation domain. However, fluent always pops error like 'divergence detected in AMG solver'
I am quite sure it's the problem of UDF. Because I tried to use kinetic theory to calculate mass transfer and perfect results can be got. Since I have almost 1000 lines of code in the UDF. I don't know how to check the error.
ps: UDF can be compiled and loaded without any error.
I put the residuals and error screenshot below. Can anybody give me some suggestions about what might be the reason for divergence?
Best.
weiqiang