Quote from kostia00:
Which version of TWS are you using?
The "modify mismatch on field #6205" error message really means "IB does not support changing RTH-only flag on a submitted order".
I really need to comment on a couple of issues:
1) these error messages are meaningless to everyone other than IB developers. Really need to add better error descriptions.
2) Though the above error is not related to the API, its similar to what you get via API. When using the API, there should be no popup error messages ever (as others have already mentioned)