We were also seeing this internal error message. It was resolved today. This was the message we received from Marketo Support "Our Engineers identified a problem with this issue and found a bug with the email editor. It appears as if they have released a patch for this bug and have deployed across all customer pods."
A
Anonymous
September 29, 2016
Thanks for letting everyone know Savina,
Now am not getting the server error and it is working good now. Will hope for no more server errors.