Logon Screen appears during a custom RFC call

Hello Experts,

I am encountering a strange situiation.

I have written a program which uses an RFC Function module for parallel processing.

But the problem is that a logon screen for some external system appears asking for my userid and password.

But the the real problem is that once I terminate this execution and re-run the program,

no logon screen appears and the program executes succesfully.

Invocation of this logon screen is random and I am not able to guess when it will appear.

Kindly, help me with this issue.

Thanks In advance.