CiviMail doesn't save when using Checksum event links
I just had this exact same problem.  (I'm on Version 5.25.0, posting over a year later, so an upgrade would not have been the answer)
What &quot;fixed&quot; my problem was to change the order of the parameters so that the checksum parameter was first after the ?
https://DOMAIN.com/WEBFORM?{contact.checksum}&amp;cid1={contact.contact_id}
instead of:
https://DOMAIN.com/WEBFORM?cid1={contact.contact_id}&amp;{contact.checksum}
It is odd that the checksum token inserts &quot;cs=&quot; before it, which breaks the URL validation.