Hi there @nmiletic,
I had a look at the site you mentioned specifically, https://calgaryneuropathy.com/, and gave it a test run, and it worked fine for me.
If the issue is happening intermittently, then it is most likely an issue with a security plugin you are running on the site, or security settings on the web host.
Please check this knowledgebase article under that error you are seeing for a list of known caching / security plugins that can cause this error:
https://kb.mailpoet.com/article/271-error-messages-when-subscribing
As suggested in that article, one easy way to get around this is to add a captcha to your sign up form – as a bonus this provides protection from spam bot attacks on the form, which can cause your account to have its sending service paused if an attack like this happens. It looks like this site is using a MailPoet sign up form, so you can use the built-in MailPoet catpcha which can be enabled under MailPoet > Settings on the “Advanced” tab.
One other potential issue is that the activation key might be dropping its connection from time to time, requiring you to manually re-verify it on the MailPoet > Settings “Key Activation” tab. If you are seeing this type of behaviour, please consult this knowledgebase article for assistance:
https://kb.mailpoet.com/article/398-what-should-i-do-if-my-mailpoet-key-is-deactivated-on-its-own
