I've tested out the links in the previous emails, and a couple of new ones, and we're no longer seeing the error, so that at least gets that aspect of the site working again. Thank you!
I would still like to figure out why the true links aren't being sent in the emails, though. For example, the email contains this link: https://imperialgunners.com/forums/index.php?app=core&module=system&controller=redirect&url=https://imperialgunners.com/forums/index.php?/topic/2038-request-for-igc-deployment/%26do=findComment%26comment=61385&key=7d1e9bd543ceebbe549fa0625bbf561baf0430c34f33849d6850fc9e4e2220bf&email=1&type=notification_new_comment
But becomes this in the browser after clicking on it:
https://imperialgunners.com/forums/index.php?/topic/2038-request-for-igc-deployment/page/33/#comment-61385