I suggest to set the IPN link in the plugin settings to https://www.store.com/Plugins/PaymentPayPalStandard/IPNHandler and give it a try.
My (newer) version of the plugin does not have this setting, so I can't check it now, my plugin sends the store URL with the callback address by default so this setting was left out.
Another thought:
If you can set the whole page to SSL only by setting 'Force SSL for all site pages' to true. (a pure SSL site is becoming a good point nowadays in SEO rating anyway) then I would suggest to include an URL rewrite rule in your server configuration to direct all non-SSL traffic to the correspoding https address. This would perhaps direct the paypal callbacks to their ssl variants even if you can't set it to https.
I am running IIS 10 on a windows server 2016, this is what I use in the web.config for the URL rewrite: (you should have the URL Rewrite module installed first) insert this between the system.webServer tags.
<rewrite>
<rules>
<rule name="HTTP/S to HTTPS Redirect" enabled="true" stopProcessing="true">
<match url="(.*)" />
<conditions logicalGrouping="MatchAny">
<add input="{SERVER_PORT_SECURE}" pattern="^0$" />
</conditions>
<action type="Redirect" url="https://{HTTP_HOST}{REQUEST_URI}" redirectType="Permanent" />
</rule>
</rules>
</rewrite>
PS. The good point in this error that your payments are still processed, you are just not getting the markings 'Paid' which you can do manually in the meantime. In our case with the direct module we were facing complete blackout.
My (newer) version of the plugin does not have this setting, so I can't check it now, my plugin sends the store URL with the callback address by default so this setting was left out.
Another thought:
If you can set the whole page to SSL only by setting 'Force SSL for all site pages' to true. (a pure SSL site is becoming a good point nowadays in SEO rating anyway) then I would suggest to include an URL rewrite rule in your server configuration to direct all non-SSL traffic to the correspoding https address. This would perhaps direct the paypal callbacks to their ssl variants even if you can't set it to https.
I am running IIS 10 on a windows server 2016, this is what I use in the web.config for the URL rewrite: (you should have the URL Rewrite module installed first) insert this between the system.webServer tags.
<rewrite>
<rules>
<rule name="HTTP/S to HTTPS Redirect" enabled="true" stopProcessing="true">
<match url="(.*)" />
<conditions logicalGrouping="MatchAny">
<add input="{SERVER_PORT_SECURE}" pattern="^0$" />
</conditions>
<action type="Redirect" url="https://{HTTP_HOST}{REQUEST_URI}" redirectType="Permanent" />
</rule>
</rules>
</rewrite>
PS. The good point in this error that your payments are still processed, you are just not getting the markings 'Paid' which you can do manually in the meantime. In our case with the direct module we were facing complete blackout.
Hi, we have alredy installed and ssl, and check 'Force SSL for all site pages' to true, we have added also your rewrite rule and made a test, but it fails too..
Don't know what it can be, I have seen something that paypal has changed their paypal ipn postback url:
www.paypal.com/cgi-bin/webscr to ipnpb.paypal.com/cgi-bin/webscr
But because all was working fine days ago, I think this is already implemented in the plugin