On Sun, May 9, 2021, at 12:17, Yuuki Harano wrote:
> Does this work for you?
> 
> (put 'none 'modifier-value 0)
> (setq x-hyper-keysym 'none)

Yes this works perfectly for ISO_Level5_Shift. Thank you very much.

Sadly I stumbled upon another problem:

ISO_Level3_Shift combined with other modifiers is also partly broken. E.g. when using C-ISO_Level3_Shift-w (ISO_Level3_Shift-w is & in my layout) it just sends '&' instead of 'C-&'. Strangely enough the same is not true for 'C-ISO_Level3_Shift-a' which sends 'C-]' as it should. And 'C-IL3-u' sends 'ESC' instead of 'C-['. I am thoroughly confused.