By itself, that doesn’t mean much. My id in Atlassian identity was created when the service first came on line, so that’s the “original format”. A new account will no longer have a : segment in the front. If you think there might be something about your account id, it would be best to pursue that with developer support. Privacy policies prevent me from just going from this public thread to “look you up”. We need the traceability of a support case. As such, it’s pretty typical for auth problems to end up needing to go to support.
However, this isn’t auth and I trust how you obtained your own id. Per my experiments, a wrong id would give us a different error. I wonder if it’s a configuration problem. Could you try using the 2 custom field ids from my response? Both are out-of-the-box custom fields (weird, right?) so they exist in every new Jira instance. Maybe there’s something about your new custom field or how it’s configured for that request/issue type that is the problem?