Hello,
Thank you both for your help.
@SwiftExpat : the message is right there after launching the exe file. As a standard user, I get some dumb conversion error message like 'cannot convert null to integer' whereas with the admin user, it just opens the window (no error).
@Keesver : Thank you, I will contact the hosting company. On the user's citrix machine the program is sitting on a network drive (the application server is another server than the citrix machines). Maybe that changes anything?