As SD stated it is probably mod-security.
The forbidden message is generated by your server.
Meaning you server is not allowing changeuser.php to run.
it could be you have other issues but first is to eliminate the obvious.
Is mod_security blocking the script.
Which is something your web host installs.
Sometimes you are allowed to modify it or disable it.
Then file folder permissions as I stated before.
But you also eed to check the owner/group for each file/folder.
I have seen it happen to myself after a host upgraded the server when I had the correct permissions set on the file/folders.
But the owner/group was set to something that I did not have access to.
If all this is way over your head.
You may need to hire someone to resolve.
They probably will need full access to the site not just ubb, but your host control panel as well and possibly your host trouble ticket system, to submit a trouble ticket for them to resolve.
Or simply change hosts.
Since we don't know the url or who your host is.
