Hello,
Next time this happens could you hold down Control+Shift+J" on a PC or "Command+Option+I" on a Mac and when the side window opens, take a screenshot of the error console ( make sure the side window is on the 'console' tab)
This might give us some insight into what error is occurring. Also if you retrieve your User Key from the Form Mule drop down, we can check the backend for errors.
I would also make sure at the time you are running Form Mule, that you are only logged into one google account. (multiple accounts open can cause permission errors). Thanks!