vbkun on "wp-login erratic forbidden error"

I'm experiencing problems with the login page, having sometimes a 'forbidden' error. The thing is, it does not always occur, sometimes after login it goes ok to wp-admin, sometimes I get the forbidden error.

What I tried so far:

- Reverting to default theme.
- Disabling all plugins.
- Changing wp-login.php permissions to 775.
- Disabling "SecFilterEngine" and "SecFilterScanPOST" at wp-admin folder.
- Creating a new User.

What's weird about this:

- When it happens, if i use browser's back to go try log again, the error persists, no mater how many times I retry. If I do go back and refresh the page I manage to log in without the error.