Mac OS 10.7 Server won't mount DMG's after opening and closing - "Resource Temporarily Unavailable"

I am running Mac OS 10.7.3 Server on Mac Pro 2 x 2.66 GHz 6-Core Intel Xeon with multiple users editing on Avid.

DMG's containing raw camera footage are stored on the server, mounted by connected users on their desktops to allow Avid to access the enclosed data within the DMG's. Recently we've started encountering a problem opening the DMG's after they've recently been opened and closed. After opening and closing a DMG. when we try to open it again from the same user/connected computer or any other user/computer we see an error message saying "The following disk images couldn't be opened" NAME OF DMG "Resource temporarily unavailable". This happened to several users with several DMG's.

We have since restarted the server, which appeared to fix the problem for about a week and now we are having the same issue again.

The only changes made recently have been an update to version 4.2 of our ISIS 5000 which all of the Avid systems edit off of, but I don't beleive that to be the cause of the DMG opening issue. I suspect it might be caused by the Pram still thinking that the DMG's are open when they've already been closed.

Not sure if this is relevant, but we had been running only Mac OS 10.7 systems connected to the server but within the last few months we've been adding several more systems including those with OS 10.8.

Has anyone else encountered this issue? Does anyone know the fix to this problem?

Thanks for any and all help!