VittorioTomasi yes I tried accessing the url but it's not giving any more clue.
It looks like the route (url) isn't properly registered. Possible reasons could be another extension registered a route with the same internal name, but I checked the other exts and it doesn't look like it's the case.
Another option could be that the extend.php file of the extension was somehow damaged. Could you check the file <flarum>/vendor/fof/linguist/extend.php has the same content as https://github.com/FriendsOfFlarum/linguist/blob/0.4.1/extend.php ?
Maybe try removing the extension with composer remove fof/linguist and install it again.
Remaining options are that your webserver config causes the url to be altered before it reaches Flarum and Flarum tries to answer a slightly different url than the one we see. But it would be strange if it happened only for that extension, and the url we see in the error message looks like the correct one.
How did you install Flarum ? Only from the official docs https://flarum.org/docs/install.html or did you use another tutorial ? Did you point the root directory of your VirtualHost to the public folder (as expected with a normal install) or did you follow another procedure to hide the public folder from the url ?
EDIT: I just tried installing linguist on a brand new beta 10 install just to make sure it's not broken after last update. Version 0.4.1 works fine for me.
Another option you could try is disabling the other third-party extensions and see if it works. If it works, then you can re-enable the extensions one by one until Linguist breaks again to find out which one is the culprit.
