I deleted everything in inc/plugins but I'm getting all the exact same errors at the top. Tried having the no_plugins line in settings.php at 1 or 0, either way same errors but I can access the board and all the posts and everything works, just with all the errors at the top.
Yes, you will do, because a variable is in the template that isn't being defined by a plugin because the plugin isn't currently running. That error isn't the issue, you need to establish which plugin is causing the white page, by re-uploading them one by one. Also make sure no_plugins is set back to 0 so they'll actually load.
Ah I see, thanks so much for all your help! So I went one-by-one and it seems the Style Usernames (styleUsernames.php) file is the one causing the blank white page. With that in mind, should I be looking to purge any related files to that plugin now to get rid of the errors?
Possibly, if it's
this one it hasn't been updated for getting on for 6 years.
Ah I see. Well all seems more or less fixed now, thanks again for all the help!