Hi,
the remote control setup in the webif will change the kernel internal keymap. The VDR config will not be change and must not be change.
So there is no need to train the remote by the VDR itself.
To check, if the new keymapping is working, you can use the tool 'evtest' on the console. It will show on every keypress the keycode. So you can check the keycode for the exit key.
By the way: What does the exit key should do? Do it should do the back command?