I have a CLD-2950. I can turn it on, tray open / close works and even playing an LD. But: If I close the tray without an LD or with only a CD inserted, the laser slider moves all the way to the middle and keeps banging against the spindle until I cut the power.

I suspected one of the three limit switches but they seem ok. Any ideas?

Visual inspection isn't good enough. You have to measure the limit switches to confirm they're OK.

I did, they are OK.

You have a dead Supercapacitor on the main board. There is a repair video somewhere on youtube about this issue.

This is the video I mentioned earlier.
The banging starts at 1:50.
Is this the issue?

Yes, that's it. I'll have a look at the capacitor.