GOA-configured EWS mail account disappears when opening Evolution with unreachable EWS server
Short version: MS Exchange mail accounts may disappear from the Mail view if Evolution is opened when your Internet connection is down. It can be brought back by restarting Evolution background processes.
I have a few email accounts managed by Gnome Online Accounts, one of them being a Microsoft Exchange account. It is set up to sync Mail, Calendar, and Contacts.
Today, I opened Evolution when my internet connection was down, and my MS Exchange email account disappeared from the Evolution's Mail view. My other mail accounts were still visible. Reconnecting to the Internet didn't bring back the MS Exchange mail account, but the account still showed up in Settings->Online Accounts, and Evolution still showed Calendar and Contacts information from that account. It was just the mail account that disappeared from Evolution.
Closing & reopening Evolution doesn't solve this, but thankfully restarting the computer did, though it placed the account at the top of the list in the sidebar. I assume just restarting the correct background Evolution process(es) is enough to fix this too, but I didn't know which ones to pick, so I just opted to restart the whole computer.