Dear All, @djelectro
I had the same problem, but I already solved it.
Researching and reading thousands of topics in the forum, I couldn't find the solution.
The problem is that version 1.17 of osTicket that is available for download has a blank .htaccess file (it only says "Deny from all").
I copied and pasted this:
<IfModule mod_rewrite.c>
Rewrite engine on
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} (.*/api)
Rewrite rule .* $ %1/http.php/$1 [L]
</IfModule>
And with this OAuth2 worked.
Check that the .htaccess has that configuration.