The error was a migration issue upstream. More info is at:
https://github.com/monicahq/monica/issues/1626
https://github.com/monicahq/monica/pull/1673
https://github.com/monicahq/monica/issues/1667
https://github.com/monicahq/monica/issues/381 is the security concern.
Using the ID can allow people to get a very good sense of how many users are on the system and the amount of contacts. It also is information leakage because I know all the URLs for every contact.
I think using a hashid based on the contact id, the user creating it and maybe another factor would work great.