User Jagster encountered a LoadError: cannot load such file -- topic_list_item_serializer during a Discourse upgrade in a single-container setup. Initial speculation pointed to the Microsoft Auth plugin or PostgreSQL updates, but pfaffman and merefield clarified the issue was unrelated to plugins or PostgreSQL. merefield identified the root cause as a recent breakage in the Topic List Previews Sidecar, a compatibility issue that had resurfaced after a long period of stability. After investigating, merefield confirmed the issue was resolved, and Jagster confirmed the fix worked after rebuilding. The discussion also briefly touched on Discourse’s handling of empty posts, referencing a meta topic where merefield noted the behavior was previously discussed. Jagster humorously acknowledged learning something new about quote formatting counting as empty posts. No plugin disabling was needed — the fix was applied upstream by merefield.
I just tried do upgraiding and it failed. Only errorlike I found was this:
I, [2024-08-13T05:52:05.670992 #1] INFO -- : > cd /var/www/discourse && su discourse -c 'bundle exec rake db:migrate'
Plugin name is 'discourse-microsoft-auth', but plugin directory is named 'discourse-plugin-office365-auth'
rake aborted!
LoadError: cannot load such file -- topic_list_item_serializer (LoadError)
Single container setup, nothing fancy.
Is this something I can do about it?
You can try updating the url for the Microsoft plugin for starters, though I’d not think that it’ll cause/fix the error you describe.
Click the url you have now to get redirected to the new url.
Well, I still trust plugin itself will fix it because the plugin and its old settings is the reason 
But you are right, it has nothing to do with this issue and it pops up quite often.
My bet is it has something to do with pqsl, again, because it did something that could be updating something.
There was a recent update to postgres, so if you have a two container setup you need to rebuild the data container.
Oh, quote is counted as an empty message. I didn’t know that — everyday something new.
Why would this have anything to do with an Auth plugin?
Are you sure it’s not related to another plugin you are installing?
btw, the file still exists, so it’s probably something to do with the way it is referenced:
yeah, that came up in that empty post feature request.
It has nothing to dp with auth-plugin, and I didn’t say so. It just happened been there. I didn’ bother clean it because I was so sure practically everyone knows that and see what was the real error.
Well, I was wrong.
I don’t know what causes that. But when rebuilding shows to me acts of postgres updatingwise, and error shows to that direction (well, I’m not a pro) and there is no another error, so?
I knew the whole empty topic thing was bad idea 
Well, seriously — I’ll move to waiting mode and there is no need for me to try disabling plugins, right?
HOOOOLD the front page - I think this is a failure in Topic List Previews Sidecar.
Looks like after a long spell of compatibility, something just broke.
I’ll take a look.
@Jagster this should be fixed now.
You can rebuild with the Topic List Previews Sidecar.
Yeah, you did your magic and it worked. Thanks!