I am having the same issue with all of my powerapps.
"Hmmm We couldn't sign you in", Tried clearing cache, cookies etc. and no luck in IE.
If on opening PowerApps you are recieving the "Hmmm we can't sign you in..." error and you are using IE, try using Chrome, its the only work around I can find.
To confirm: all other programes in Office 365 online open for me in IE apart from PowerApps... I have just resulted in opening PowerApps in Chrome because I have work to finish and bills to pay.
I've had the same issue going on. Was there ever a resolution? It seems like it is just web.powerapps.com that has the issue so I can get to the studio part of the site. I just can't get to the main site menu to adjust connections.
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