I'm having a the exact same problem too.
Digging into it, I discover that the Salesforce Reports connector in the PBI Service (classinfo = report-detail) is also failing and when I try to reconnect with the standard OAuth2 connector, it just hangs.
However, in the PBI Desktop, I get problems with both the Reports connector and the Objects connector. I opened the Data Source settings dialog in PBI Desktop, and clicked Edit Permissions on the SFDC connector, and attempted to Edit... the credentials. Sometimes it tells me that the comnpany's authentication service is down, and on other occassion it accepts the credentials. That's weird and could explain why it's taking so long because the credentials are only accepted some of the time - otherwise, it has to create a new connection... I've seen this type of thing happen in the past when the SFDC logon API has changed and the return value isn't handled correctly by the calling code.
Anyone else tried to dig into it?
Thanks, Geoff