The same happens to me!

Oops! I seemed to have revoked access. Works fine now. Sorry!

Same for me ..... no chance to upload anything.

Hi Mark, 

Welcome to Box Community, sorry for your trouble and let me assist! 

If you've completed these troubleshooting steps and the error still persist.

Please make sure that you're using  account.box.com instead of account.box.net and try again.

Hope it helps!