DVD-RAM burner cannot open cartridges

[qanda]This thread is about the Panasonic LF-D321. Click here to see full specs[/qanda]I have this nice burner LF-D321 for 6 years already. I use it once in a while just to copy my most important stuff on DVD-RAM type IV disks (i.e. double-sided in a cartridge).
But recently it just refuses to accept the cartridges. You push the tray, then the cartridge enters the device, and after a second or so it opens again and rejects the cartridge.
When I put a non-cartridge disk, the device can read it without problem. So I suppose it might be a mechanical problem, the device not being able to open the cartridge window.
In order to check this theory I taped the window open and tried this way, but the result was the same.
Any advice is highly appreciated because my most important data is on these disks.
Yes, I know, that there is always this final solution to open the cartridges, read the bare disks and copy their contents to somewhere else, but it would be nice to find a more elegant solution and continue to use my cartridges. I am not afraid to open the device because there is nothing to loose in this situation, but I don’t have clues what to look for.

Thank you very much !