Regression: Old token stopped working (Hashie::Array)

We have an imposter token which was generated when our GitLab installation was young. The following metadata about it was collected before the upgrade:

{"id":11,"name":"imptoken","revoked":false,"created_at":"2017-07-06T08:04:37.736Z","scopes":["api"],"active":true,"expires_at":"2032-01-01","token":"RZm-redacted","impersonation":true}

After upgrading to latest omnibus release, 15.4.2-ee.0, this fails with a backtrace. (It is possible that we have not tried to use this token since the upgrade from 15.3.x.)

irb(main):001:0> token = PersonalAccessToken.find_by_token('RZm-redacted')
Traceback (most recent call last):
        4: from (irb):1
        3: from app/models/concerns/token_authenticatable.rb:34:in `block in add_authentication_token_field'
        2: from app/models/concerns/token_authenticatable_strategies/digest.rb:12:in `find_token_authenticatable'
        1: from app/models/personal_access_token.rb:115:in `set_default_scopes'
Psych::DisallowedClass (Tried to load unspecified class: Hashie::Array)

Since I don't know if there are any other tokens made by other users which have stopped working, I wonder if there is a migration step I have missed in the release notes?