Modify vebp-ezeit using user exit

HI I tried updating vbep ezeit using userexit_save_document_prepare.

Its working fine most of the times , Have user UPDKZ as U wherever i am changing EZEIT in XVBEP.

But sometimes 1 out 100 it throws an error V1 506 Schedule line &1 &2 is missing in table YVBEP

I am not able to understand or figure out why it is throwing the error. Very

tough to replicate the error.

PS: I have not modified YVBEP table in the exit. Is it because of that ? DO i need to change VBEP structure in this exit ? Any

other exits that might help ?

Any help will be appreciated.