Very happy it all worked out! It would be terrific if you would post the solution here (in English) in case this happens to someone else. Thank you.
Here’s the best I could do. I left out the stuff about not being a miserable schwein and not following directions.
Step 1:
The solution requires two stages, since you first have to get from error 1000 to 1001, and then revive the X1D in the last step. But first things first. So if the error 1000 comes with a firmware update, first of all: keep calm. In the first step you tap five times (5x) on the triangle symbol with an exclamation mark and additional options appear. The option you need is "FW Update Retry". The camera now tries to install the firmware again. If everything goes according to plan, this attempt should end in error 1001. This would also complete the first step. If you are already confronted with an error 1001, you can of course start directly with step 2. In this case you should be able to skip step 1 and still be successful.
Step 2:
Remove the battery and plug it in after a short wait (about 10-20 seconds). When switched off, press the bottom of the 5 buttons next to the touch screen of the X1D (the button with three horizontal lines), keep it pressed and switch on the X1D. Once the X1D has started and the "normal" menu appears on the display, you can release the button. In this step, the X1D was completely reset and the current firmware should be the last version. In my case that was the FW 1.19.0. Thus the X1D is again usable and one can start the firmware update (with me it was the version 1.20.0) again and this time everything should work and the X1D should display the message that the firmware update was successful and the battery should be removed.
Note: One should always follow the instructions of Hasselblad for the firmware update. For example, not to plug in the USB cable during the update, to charge the battery sufficiently beforehand, etc ...
The only downside after reviving the X1D50C is that you have lost all custom settings as the X1D has been completely reset. But that is okay, because you saved sending off the X1D.
And now I'm going to test the new features in firmware 1.20.0 of my X1D50C