Problems sending Mails with Mailpoet, "Request error not JSON"


we are using mailpoet for quite some time now, experiencing issues is a novum. Hope we get this sorted.

On trying to send a preview mail with phpmail we receive an error message saying "Request error not JSON". When sending the actual newsletter from the index page, nothing happens, even waiting over night.
I followed the suggestion to solve this issue (increase server memory to 128M), but no way to send out newsletter or even previews.

When i switch "sending with" to sendgrid, i get the message:
"request error not JSON:
<b>Fatal error</b>: Call to a member function __construct() on null in <b>/usr/www/users/musikzr/wp-content/plugins/wysija-newsletters/helpers/email.php</b> on line <b>89</b>

Since the newsletter contains some X-Mas offers for our clients, it would be great to find a solution the next days.

Best regards,
Sebastian of Musikzentrale.