We upgraded to the latest Discourse build; no major plugin installed, other than the fact we have a subdirectory install.
The error I am seeing in the logs is the following:
failed to handle exception in exception app middleware : PG::UndefinedColumn: ERROR: column categories.suppress_from_homepage does not exist
Is there some known issue?
(Edited this as before I thought it was a subdir error)
Still investigating this, as it seems to happen only in one of the two forums I upgraded.
I have created that column via psql for now and that brought the forum back online - unfortunately I am not familiar with when/if that column was removed.
Run db:seed_fu after the import, that will fix it.
Not too fond of this delayed-drop column mechanism, it haunted us too many times… 
Thank you Michael, this has helped! Also for the future!
Looks like it was very recently:
Heading “Suppress category from latest”.