CiviMail doesn't save when using Checksum event links

Running CiviCRM 5.10.4 and Drupal 7.65.

Trying to send out a bulk email through CiviMail using a checksum link for an event registration.

However when I insert the checksum link, I get an error message when CiviMail tries to autosave, or when I try to manually save:

Sorry an error occurred and your information was not saved

This is the checksum link I’m trying to insert

https://DOMAIN.com/civicrm/event/register?reset=1&id=89&cid={contact.contact_id}&{contact.checksum}

I’ve also tried with:

https://DOMAIN.com/civicrm/event/register?reset=1&id=89&{contact.checksum}&cid={contact.contact_id}

Have tried testing to save with every move I make when making the email, and have isolated the issue to only happening when I try to use these links (whether it’s an image linked to this URL, or text).

I’ve checked and that is the correct event ID in the URL. I’ve tried inserting the link in the source code and in the WYSIWYG editor but neither are allowing me to save. I’ve tried writing out the URL in notepad and copying over from there in case any formatting is being pulled through somehow, but again no luck.

Any idea on where we’re going wrong or how to fix this?

Cheers

EDIT: On more investigation, it allows me to save the email when I use the following link. However this creates an extra “cs=” in the URL so the checksums don’t work….

The URL I’ve used this time is:

https://DOMAIN.com/civicrm/event/register?reset=1&id=89&cid={contact.contact_id}&cs={contact.checksum}