hand_coding on "[Plugin: MailPoet Newsletters (formerly Wysija)] "Oops! There is a problem with this form""

I continuously experience this problem without finding any solution.
Anytime I try to subscribe without being logged in, the response is the same:

Oops! There is a problem with this form:
textStatus:parsererror
errorThrown:SyntaxError: syntax error
responseText:

Followed by the blog home page on a red background as the responseText.
This happens whatever method I use (php, iFrame, …).

screenshot1
screenshot2

This error doesn't show up when logged in and the subscription goes fine.
It looks like the error reported on this post a year ago but I couldn't find any reference to it on the "Knowledgebase".

Any clue?
Please help.

Fabrice

http://wordpress.org/plugins/wysija-newsletters/