Error when selecting a category in 3.0.4

Hi we upgraded from 2.5 to 3.04 on a site and when we create a new product and select a category we get the following error.

Fatal error: Cannot use string offset as an array in /home/XXXXXX/public_html/var/cache/templates/admin/%%E4^E42^E4252AD1%%mainbox.tpl.php on line 55

Version 3.0.4

Have you tried clearing your cache? To do so, append your admin URL with ?cc like so:

I sure did… worked like a charm