As of 9/25/18, this issue was resolved by working with Pressbooks to update a plugin re-enable cron.
Users getting 502 and 504 errors in Pressbooks
In Fall 2018, some users reported receiving 502 and 504 errors in their web browser when attempting to view or edit Pressbooks content. This issue is outlined in [Link for document 86021 is unavailable at this time.].
As of 2/11/2019, this issue is no longer present.
Collapsible Sections not working in Pressbooks
Links to different content within Pressbooks via anchor tags do not work consistently between different web browsers. Specifically, in Firefox, links to anchor tags which are contained within a collapsible section (underneath a header) will often display the bottom of the page the anchor tag is on rather than the content where the anchor tag is located.
As of spring 2020, this issue is no longer present.