Today, I tried to narrow down the problem what's really going on with my code and found out something interesting as follow:

In the clsB there is a function called AddNew.

public sub AddNew(byval objG as G.clsG)
dim str as string

str = objG.someproperty

end sub

On the client site, when it calls this function which is located on MTS server, the error will be occured here. Once, I took the line which consist of objG out, the program just got through it. Any ideas what's going on around here? Thanks for all opinion and helps.