Well, I installed Ndless 3 to my Nspire today. Now the screen looks like that.
Details: I put a non-cx cas gbcemulator on my calculator and clicked it and then the screen froze. Then I removed the Ndless-map with the resources.tns with the Student Software on my computer.

Apparently that was a stupid thing to do:

Do not delete ndless/ndless_resources.tns as long as Ndless is installed, as it can cause a reboot-loop that can only be stopped with a complete reformat & OS reinstallation.

I've already tried all this:

If the device is a TI-Nspire CX or TI-Nspire CX CAS, begin by holding down the reset button. With the reset button held down, press and hold the [Doc] [Enter] and [EE] keys. Release all keys at the same time. The device will boot up as in the previous reset, but will display a menu with the following options. Please try options 1, 2, 3, and 4 in that order.

1. Cancel - The most non-intrusive reset option; no changes are made, the unit restarts.
2. Delete Operating System – The operating system will be deleted without deleting any documents.
3. Delete Document Folder Contents – This will delete the user’s documents, but will not delete the OS.
4. Complete Format – This will delete the OS and all documents for a total erase of memory.

I've tried with installing the newest OS (both from their homepage and the safe copy on my computer) but

I haven't tried to remove the battery since I only have a screwdriver for the upper ones.

So, what's next?