Good news, I believe I have found the issue and the solution as well. For some reason, the metadata stored for the related posts pro plugin gets unserialized and corrupted, and returned as a string instead of an array. There might a filter or some sort of function doing it during the save process. I decided not to investigate it further, it would probably take hours, and the solution would be no different.
You will have to make a minor modification on the plugin code to resolve this. Open up the wp-content\plugins\related-posts-pro\backend\metaboxes\default-content.php and scroll to line 108, which is:
$rpp_data = get_post_meta( $post_id, 'rpp_data', true );
..just below that line put this:
if ( !is_array($rpp_data) ) $rpp_data = array();
Save the file, and it's done. This will reset the array in case of corruption. After the initial issue, this did not re-appear after, and the changes were saved correctly. I'm of course including this in the next release, so you don't have to worry about it.
I've also investigated the email issue. I suspected the headers might be set to plain/text instead of text/html, but everything was fine. I've checked the final email headers as well. My guess is that some servers might require some additional headers or other information to correctly recieve HTML mails (?), I have no idea honestly. Instead I've decided to convert these mails to plain text, and skip the HTML overall. There is no need for it anyways, as there is no styling, no images, just simple text information. The notification mail for this ticket should be correctly parsed by your mail client, or at least I hope so.
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