When choosing to log in with a Microsoft account, a second window opens and shows a plain gray background, after a few seconds it says:

Failed to load the page.

Please check your internet connection and try Reload again.
Error message: ERR_NAME_NOT_RESOLVED (code=-105).

Pressing "Reload" has no visible effects, closing the window shows the launcher with the two login options.

Internet connection definitely available. Account is migrated to Microsoft.