At some point, a newer build of Opera started claiming keyboard shortcuts that are not actually tied to features. These shortcuts interfere with Microsoft Outlook. Opera is claiming Ctrl+Shift+1, Ctrl+Shift+2, Ctrl+Shift+3. I am not sure of the other shortcuts that are being claimed, there may be more.
Even though they are claimed they cannot be found in Opera's settings to be reassigned or disabled.
I am using Outlook 2016 with Opera 62 (stable) & 63 (developer) on Windows 10. Both browsers are causing the issue. I do not get a response from Opera if I enter Ctrl+Shift+1, for example, but a web search shows it used to be the "Manage view for search panel" shortcut. Source. I really appreciate a fix for this bug, because it only introduces an annoyance in a browser that is improving all the time.