i've been able to get in without that error since late last week. so let's look at two or three things:
1) you are getting the error message when you try to go to http://web.powerapps.com/...
2) if so, have you hit F12 to see what the network sniff is telling you?
if the sniffer tells you the error message is actually an internal server error, then the problem is strange because one would expect everybody to see that 500 status code. therefore, it's more likely that it is local to your machine or network connection. even more likely something went wrong with your PowerApps subscription.
Have you tried coming in to PowerApps through the 0365 portal? that way, you should already be authenticated, and if your O365 tenant has PowerApps, you should not see any issues due to subscription issues.
