I wanted to add this here in case anyone was having this problem and looking for answers.
I think we now have a cause and a solution.
What we found is that on some sites the emailedIssuesList row in the wfconfig table had became too large from recording hashes of previously emailed scan issues. Reinstalling Wordfence fresh (where the data and database tables are removed) was fixing the issue but this lets us know exactly where the problem was.
There are 2 ways of fixing this.
1. If you are comfortable using MySQL queries you can issue this command using PHPMyAdmin in your hosting control panel:
delete from wp_wfConfig where name = ’emailedIssuesList’;
2. If you are not comfortable with running queries do this:
Remove and reset Wordfence, using the Wordfence Assistant plugin to remove all tables and data and then reinstall the plugin.
We are looking at a solution to prevent it from happening again and hopefully will include it in the next release.
Thanks always for being patient as we worked diligently to solve this for our customers. Thanks as well for reaching out and providing valuable information to help us solve it.
Thanks, stay healthy, and stay safe!