Problem with dynamic action

I've created a new applicant action called 'X1'. The first screen in the action is for infotype 0002. After saving infotype 0002, a dynamic action is called which open infotype 0021 in LIS1 mode:

1. P-PSPAR-TCLAS='B'

2. P-T001P-MOLGA='IL'

3. I-LIS1,0021

A change made in infotype 0021 triggers a dynamic action. The last row of the dynamic action is: I-INS 9100,06. Even though the dynamic action is called, the screen of infotype 9100 doesn't open. After some debugging it seems that it fails in the FORM 'END_OF_SCREEN' in MP002100 on line-IF pspar-massn EQ space. Pspar-massn in this case is equal to 'X1'. Is there a way to solve this issue? Thanks.