Motiverkennung funktioniert seit Update 11.4 nicht mehr

"then tells me a little later that an unexpected error has occurred."

This is due to a bug in the graphics driver or an incompatibility with LR.  Turning off the GPU in Preferences > Performance won't help, since Select Subject/Sky doesn't obey that setting.

So as @GoldingD recommended, the next step is to copy/paste Help > System Info here.

Please post your System Information as Lightroom Classic (LrC) reports it. In LrC click on Help, then System Info, then Copy. Paste that information into a reply. Please present all information from first line down to and including Plug-in Info. Info after Plug-in info can be cut as that is just so much dead space to us non-Techs.

In a first step I would try to reset the preferences of Lightroom Classic:   https://helpx.adobe.com/lightroom-classic/help/setting-preferences-lightroom.html

It's recommended to backup your preferences before you reset the preferences to the default settings: 

https://helpx.adobe.com/lightroom-classic/kb/preference-file-and-other-file-locations.html

If this doen't help please try to switch off the GPU support from the Lightroom preferences and check if that helps to fix the issue.

Go to Lightroom > Preferences > Performance tab > Uncheck "Use Graphics Processor" > Restart Lightroom.

https://helpx.adobe.com/lightroom-classic/kb/lightroom-gpu-faq.html#troubleshooting

https://helpx.adobe.com/lightroom-classic/kb/troubleshoot-gpu.html