Had the same problem Alien, dove me crazy. I think I have your answer (from Adobe). Worked for me...
A - GoLive handles action code in 2 different ways...
write all the code into your page
import an external script library
The default behavior in GoLive 4 and 5 was to write the code into you page. In GoLive 6, the default was changed to use the external script library, because that method works better with components and templates, and is generally more efficient.
If your page is working locally, but not online, then there is probably a problem with the path to your external script library. By default, at the root level of your site, there should be a folder named "GeneratedItems" and inside this is the "CSScriptLib.js" file. That is your external script library. Follow these steps to troubleshoot...
First, make sure you've uploaded the GeneratedItems/CSScriptLib.js to the server. If it's not on the server, the browsers won't know what to do since they can't find the code. Before uploading, it's also a good idea to "flatten" the file. This will optimize the file making it much smaller, and as a result your pages should load faster. To "flatten" the script library, right click (control+click, Mac) on the GeneratedItems folder and choose "Flatten Script Library" from the contextual menu. If this doesn't fix it for you, then...
The path may be pointing to an absolute location on your hard drive instead of the GeneratedItems folder in your site root folder. This can happen as a result of working on a page using actions while not having your site window opened. It can also happen if you just aren't using a site window/file. The problem can also be caused by a special character in one of your directory names that's confusing GoLive's internal database. This could be a problem even if the directory is above the level of your root folder – even the name of your hard drive. Look for problem characters like #, $, ?, ƒ (option+f, Mac), /, \, etc.
To fix this problem, try opening your page, making sure that your site window is opened and your page is located within the "Files" tab on the left side of the site window. Select the page icon at the top left corner of your document window and go the the HTML tab of the Inspector. Try selecting the option to "write code into page". Now try uploading the file to your server. If it works now, then we've identified the problem. You can now either leave it set to write the code into your page, or change it back in the Inspector to "import script library". If you files are set up correctly, the link should be written back into your page to point to your GeneratedItems/CSScriptLib.js file.
If you've done all this and are still having problems, try posting a link to your page in the user-to-user forum so others can see if they can find what is causing the error.