403 Forbidden error when trying to create new content

So I was (or rather am) helping a friend to transfer his site to drupal 7. He set up drupal in a subdirectory to root (/test/) so we'll only change to drupal once it's really ready.

It worked great for some time, until suddenly a werid bug started happening: Whenever I tried to a create new content node the editing window would come up as expected, but when I clicked "save" it instead sent me to the administration page for creating the same content (so basically deleted everything I put in the form and sent me to the editing window in the administration section, the one that you don't see the site in the background and all that.)
When I tried to save the node from THAT page it just sent a 403 forbidden error ("You don't have permission to access /test/ on this server.) My friend managed to create a page after I reported to him, but he too was faced with the error page when trying to create another one.
He moved evertyhing to a new subdirectory (/drupal/) and suddenly everything worked fine, except for a specific content type that would not be created when I click save, but wouldn't redirect me to the administration page like before.

Now this morninng I tried to create new, simple page and it was successful, but when I tried to create that content type that caused trouble last night it threw the "403 forbidden - You don't have permission to access /drupal/ on this server" error again (obviously this time with /drupal/ and not /test/). I tried to create another content type and the same thing happened, and finally when I tried to create another simple page like before, even that caused the problem.
I still didn't hear from him, so I can't confirm weather he is experiencing the same problem as I do.

Did anyone else face a similar problem and could share how he solved it with me?

Thanks in advance!