You might try the relatively new Re-authenticate found from
Settings > Accounts > your_Murena.io_account > My Account (overview) > Settings (Additional) > Tap the solid blue section (is your ID) > top right “gearwheel” settings icon > Re-authenticate
(If you find it hard to find a Search will find posts with some screenshots.)
Removed the account, restarted the phone, added account again. (This triggered the 2FA process by the way, so no “app password” needed, contrary to what the documentation says.)
Mail is working. And now the phone shows a notification from /e/Drive saying “File synchronization / NNNN files to sync”.
Weeks later this happened again. Today I had another round of troubleshooting.
Instead of using my main password I did follow the advice from the docs this time and created an app password. It wasn’t straightforward to use it though, because the main login process would fail with it. Instead, I had to click the “use a specific server” option, which would then allow me to enter an app password. At the very least, the docs need to be updated.
Then I immediately got an error message coming from my mail application. So I entered the settings, changed the authentication from OAuth2 to Normal password, entered the main password, got another error message (?), and now it’s working.
This was far from intuitive and I wonder how everybody else manages this. Anyway, I hope it’s the last time in a long while I have to re-connect my account like this.
Eventually re-added the account using the main password, then the phone generated its own app passwords, and the remaining photos got uploaded while I wasn’t looking. Email is also not giving auth errors anymore.