How do I solve 40277 error, UNDO statements ?

Hi,
That error is often in the past weeks. It's a welding robot and in the middle of the program it stop's with that error and it's UNRECOVERABLE, so i have to reset the system. I don't know what UNDO statements means or UNDO clause. I have other 3 robots, they are the same, but only in this one I have this problem.
The error is:
Task T_ROB1: The program execution was stopped while processing the UNDO statements. UNDO was not fully executed. The routine ArcLStart was executing when UNDO was topped.
Actions: If the processing of UNDO takes too long, try to remove time-consuming instructions such as TPWrite from the UNDO-clause. If the UNDO processing never seems to finish, make sure any loops in the undo-statements are correct.

Thank's.