I have this problem too. I have checked the memory allocated through PHP and set it to 128MB, turned off any security plugins which could be getting in the way. Possibly related, if I go into the settings (send With) and check to see if MailPoet can send a message to me. It comes up with a error message which starts with the text: Request Error not JSON:<!DOCTYPE html>….. basically a web page. This freezes everything. If I close it I can regain control. The test email is never sent. So whatever is causing this is also related to the sending function. This does not work either with a third party email account now either. It used to.
I have reset the plugin completely. The problem remains. The odd thing is I use this plugin on a number of sites, it was working prior to the upgrade to 4.3. It is not working now. Any clues would be helpful before I take this site down completely and rebuild it somewhere else. I have not seen this problem on my other sites, but each is a little different.
Paul and whoever else happens to come across this post, I have found out what the reason was in my case. It turns out it was nothing to do with the installation. I copied the whole site to another hosting company I use and it worked fine. I have spent over 6 hours taking this apart and trying different things, however at the end of the day it was the hosting company that caused the problem, not MailPoet or the WordPress installation. In this case I was using NameCheap who seem to have introduced a ModSecurity rule that affects MailPoet installations. The odd thing here is this all worked at the point it was installed. So they introduced this block over the past couple of weeks. The reason is because of a vulnerability which was fixed some time ago. Suggest you ask your hosting company to check any ModSecurity rules that might apply to MailPoet. Once it was identified and the rule whitelisted the site worked fine. Here is the exchange with the hosting support person:
19:23 NameCheap: So it was triggered by ModSecurity rule that our technicians have whitelisted.
19:23 ME: Who put the modsecurity rule in place?
19:24 NameCheap: It is triggered automatically.
19:25 ME: If that is the case why does it not create this error on the other sites I have in your hosting.
19:25 ME: I use the same plugin on multiple sites
19:27 NameCheap: Unfortunately, I cannot answer you as it is triggered in different cases that there is no the exact answer on the questions why and when.
19:28 ME: So what is the current position on this hosting, is it whitelisted now?
19:28 ME: Is there any further action I need to take?
19:29 NameCheap: Yes, it is whitelisted already and it should work in the future. But if you have such issue again, please contact us.
19:29 ME: Can you tell me what the ModSecurity rule was that triggered this?
19:32 NameCheap: It was Access denied with code 403, CSRF vulnerability in the MailPoet Newsletters WordPress plugin before 2.6.11
19:33 ME: But that was a very long time ago, that bug has been fixed for some time. This version is up to date. 2.6.16
19:34 NameCheap: Yes, I understand. But as you can see it triggered that issue.
He’s talking about this: https://support.mailpoet.com/error-403-on-admin-ajax-php/
If that doesn’t fix your issue, just update to the latest version 2.6.17
