During log in it just said: Whooops! something went wrong!

I am new to this. I just played MC on my iOS iPad and this was the firdt time I tried to installit on my Windows PC. All went OK until the last step of login. It just says :

Whooooops! Sorry, something went terribly wrong and you probably encountered a bug. Please file a report. 

and that's it and yet I cannot login!