Wiki fails to render some pages (ImportError: No module named docutils)

Description of the problem

Some pages fail with:

Failed to render page: Traceback (most recent call last): File "/opt/gitlab/embedded/lib/ruby/gems/2.4.0/gems/gitlab-markup-1.6.3/lib/github/commands/rest2html", line 50, in from docutils import nodes, statemachine ImportError: No module named docutils

Which Group/Project (with full path) is experiencing the issue?

https://gitlab.com/catalyst-samba/samba-docs/wikis/index https://gitlab.com/xomachine/SteamForwarder/wikis/Why-SteamForwarder-needs-steam-api-headers

So far only the second page is known to Google.

Approximate date/time when the error occurred.

This was found on 2018-07-18, and appears to be always reproducible.