I suddenly started facing this issue. Upon going Catagog > Categories > Manage Categories, the output is something like this & none of the options are clickable:
I cleared the cache, ran reindexing, but nothing worked. None of the tabs on category page are clickable and hence I'm unable to modify any info in the categories. Please help.
Visual Merchandiser
. Check your Magento logs and/or server response in dev mode or try disabling that module to see if the error persists.