When I start Thunderbird (60.0) it shows a short moment the email-screen (Email/Accounts/Advanced versions) and after that goes to Box in.
There has been an update of Thunderbird 52.8.1 to 60.0 installed (using Windows 10). When I start the new version of Thunderbird 60.0 there is a short time that a screen has been showed with: Thunderbird E-mail - email address E-mail Accounts Advanced functions (geavanceerde functies)
Without any manual intervention the screen Box in (Postvak in) is showed.
It is not an error but only a strange situation.
Opaite Mbohovái (1)
Thanks for reporting this. It is a known issue. https://bugzilla.mozilla.org/show_bug.cgi?id=1484270