We verified this to be a bug with version 5.1.

For those experiencing this issue, and until version 5.2 comes out (which includes a fix for this), please contact us through our support email.
We can basically tweak a value on the database to workaround the problem, and you'll be able to access the menu after that.
If you could attach a backup of your database (Setup->Database->Online Backup & Restore) with your email, we should be able to send you a fixed file within minutes that you can restore and you'll be back in business.
We apologize for the inconvenience this problem has caused.
Regards,
Ernesto.