I installed xml.vim with AutoComplPop (http://www.vim.org/scripts/script.php?script_id=1879) annd when i open an xml (or html file), as soon as I type < , I get a "E764: Option 'omnifunc' is not set" error, and vim will keep on complaining with the same error for every character i type until i close it with a >.

After that, the tag is auto-closed as expected.