[DRE-maint] Bug#1019731: gitlab: failed to fetch comments in merge requests (500 Internal Server Error)

[Maximilian Stein]

Package: gitlab
Version: 15.3.2+ds1-1~fto11+1
Severity: normal

Dear Maintainer,

Some time after upgrading to 15.3.2 (but not from the beginning),
loading of comments in merge requests started to fail. This sounds
similar to #1019403, but I believe it is a different issue.

While loading a merge request with comments, this URI (relative to
project) fails:

    -/merge_requests/346/discussions.json?per_page=20

In the production.log, I found:

    Completed 500 Internal Server Error in 764ms (ActiveRecord: 493.9ms | Elasticsearch: 0.0ms | Allocations: 144723)

    Psych::DisallowedClass (Tried to load unspecified class: Gitlab::Diff::Position):

    app/models/diff_discussion.rb:13:in `position'
    app/models/diff_discussion.rb:51:in `cache_key'
    app/controllers/concerns/issuable_actions.rb:186:in `render_mr_discussions'


Unfortunately, I haven't found a workaround yet. Do you have any idea
how to get comments in merge requests again?

Best,
Maximilian