Thanks for your replay
Its is a User exit after up-gradation we have lot of issues we through that it is also part of it but now we realized issue with code only
To resolve the issue i copied the sy-tabix value in to the variable and that variable i used in modify statement.Because inside the loop that function module changing the sy-tabix value for that reason modify statement went into the dump.now its working fine.
Thanks,
Shiva