Not Acceptable error

Been struggling also the past two weeks when trying to send emails manually.

-Send emails to recent post done in last 2 days.

-Manually creating the email.

-Error received: Not Acceptable
An appropriate representation of the requested resource could not be found on this server.

I have deactivated, and activated the plugin. I checked for plugin conflicts, enabled debug, no success. However, when you select fewer users, it seems to work, but as soon as I say select all subscribers the error appears when trying to create the email batch.

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