JC_ProPlus this looks like an issue with an extension. It would be nice if we can identify which one it is and submit a bug report.
Could you try disabling some of the extensions until the issue goes away ?
Or if you don't have too many extension, share your php flarum info so someone else can know which extensions you have and investigate on their side ?
In the error trace I see a reference to addUpdateAlert(), which seems to only exist in fof/terms. But this doesn't look like an issue with Terms itself because terms does not access a post user.
