ALT key not working with some shortcuts

So out of the blue the alt key is not working with specific commands. Yesterday it did, today it doesn't anymore. I can't use the "save for web" command for example, also taking steps back does not work (or as in the new version "last status on/off").
When using the marquee tool the key does what it is supposed to and subtracts from the selection – so the key itself is working (which I have also verified via a browser test). The keybinds are set correctly in the options.

What I have tried so far:

rebooting, clearing cache and starting up with deleting presets, resetting workspace, switching to standard workspace, switching keybinds to standard and as last restort – reinstalling. without success.

Any idea?