Cross-Thread error but no threads!?

You are assuming there is no valid explanation, so you don't provide much information to go on. You say "calls a webmethod"; that is likely highly relevant but we need more information about that.
 
Did you look at what the debugger says about the problem? Look at the location of the error; what was your program dong when it got ther error? What function was the program in? For questions such as this, please provide information such as that.
 
Also, did you add a line of code that causes the compiler to ignore cross-thread calls, so that a relevant compiler error message is not produced? If so, then please remove that line of code.