I cloned an existing magento 1.7.2 installation on the same server with a test subdomain. The frontend seems to work, and I can login to the admin. The admin menu doesn't work however, no dropdowns, and copying url paths doesn't work either. I've searched online, and most answers date back to 2008 and suggest that it's a rights issue. So I've changed the rights of folders and files to 755 and 644, but still no working menus. The cache (var/cache) is empty. 

These menus are javascript generated. The following error message is from the console:

```
Error: TypeError: Element.addClassName is not a function

```

To be clear - the solution is not in javascript, but it's something on the server. This install works on the same server in another directory with another domain. 

Any ideas how to fix this?