Can not delete Device Group.

Hi.

I have DeviceExpert installed on a linux box. So recently I've created several device groups. Now I want to delete one of them by clicking on a red cross at the most right column of a group list, but here I have a problem. The system reports me an error message of a kind:

This device group is referred by some modules

That had occured when this device group was selected as a filter criteria in one of a dropdown lists (for example on a homepage or in a report section). I have checked all such a places in the interface and unselected this group there. But the problem persists. Logging out and in again does not fix this. It seems that the state of a dropdown selectors is saved and persists through all the sessions.

Is this the right behaviour?
How can I fix this situation?

Thanks in advance for all kind of support.

Ilya Serov.