How can I find out which shortcut is using a particular key combination?

I'm going crazy here... I'm using Divvy and it seems over time macOS is changing some default keyboard shortcuts in the system. Because on an older running installation, I could easily use the combination Opt+← and Opt+→ but I can not on a newer MacBook. It simply tells me that this shortcut is already used by a system-wide shortcut. Great. The thing is that when I go through **Settings > Keyboard > Shortcuts** I don't actually see this combination set anywhere. Also when I use it nothing happens on my system. Great. Questions: - How do I find out which system shortcut is using this combination? - How do I disable this unknown shortcut to free up my beloved key combination?