Recent versions of Joomla! have been seen to break PayPal button code.
This is because the newer versions are stripping the action attribute from the <form> tag.
You can fix this by following these steps:
- Go to the Article Manager
- Select Parameters
- Scroll down to Filtering Options, Select the front end groups (Control-Click the three groups)
- Select the Filter Type Blacklist (Default)
You may wish to select different groups.