First rule of Mount REquests:
Don't pay any attention to what media is being requested
AQ Mount REquest means that either there is no writable media in teh Media pool , or Data Protector doesn't think that there is any writable media
What is 'writable media'? Media that has a Protection of None or Expired
I would recommend that you start with this statement:
"on new week monday i insert different media"
You may have put a new media in a slot, but, is DP aware of it? After adding the media, did you do a Scan of that slot? IN this circumstance, DP doesn't knwo that you put media into a slot. A scan of that slot will update the IDB, and, maybe, fix the problem
In your Backup specification, go to the destination tab, and get Properties for the device being used for backup, and see what the default media pool is (it may say 'No media Pool Selected')
If it says 'No Media Pool Selected', go back to Devices and media, get properties for this device, and see what the default media pool is for that device
Once you have determined what media pool is being used, expand 'Devices, expand your library, single-click on the word 'Slots'. Click on the column header 'Pool' to sort the media list by Pool. Find media from the desired pool, and check what its Protection status is. It has to be either None or Expired for at least one media in that media pool