@Tornelo said in Can't change account in Sidebar Google Calendar:
If you click on the avatar and switch it just opens in a new tab rather than change in the sidebar.
Sidebar pages can't redirect from one domain to another. Doing so either shows an error page or opens the link in a new tab depending on the situation. It's possible that's what's happening here. It might be possible for Opera to fix it, but not sure.
What you might be able to do right now is goto the URL opera://settings/content/all, type google.com in the search field and click "Delete displayed data" to log out of Google and clear its storage. Then, goto the URL opera://settings/clearBrowserData and clear "Cached images and files" for all time.
Then, in a tab, sign in to Google with the account you want. Then, open up the Google Calendar or Gmail sidebar and click its reload button. That might work.