I disabled my Autohotkey script already, no change. Same goes for changing from German to English layout.
I have some keys remapped, but neither the right CTRL nor Pause key is part of that. What I did just notice, though, is that using left CTRL + Pause does the same, despite the left CTRL being remapped to be the left Windows key and the key I remapped to left CTRL doesn't do the Pause combination. Using CTRL via mapped mouse button + Pause also doesn't close the window.
The Logitech G915 keyboard *does* send a different keycode for this combination of keys, but I wasn't able to find correct code. Some software claims that Pause alone would send Num Lock and CTRL + Pause would send Pause, other software says it's unknown or undefined. Teamspeak claims it to be CTRL + Scroll Lock, which it definitively is not.
So far HWinfo seems to be the only application to react this way to this key code, though.