Cognos 7.5 cat looks grey

[rocvtict]

Hello,

we are usrng cognos 7.5 (planning to upgrade in a few months to 10) And we haven the following issue.
When several users open an .cat file with impromptu some of them get an grey screen in return. (instead of the data) Impromptu reports an error saying impromptu doesn't recognize the catalog.

We are using 7.5 admin version in combination with
app-v, windows 7, windows 2008 r2 server envirioment.

We have this problem with several departments. (differente pc's, printers)
I'm have trouble to reproduce this error because it is random generated.

Does this ring al bell to anyone?