First look carefully for differences in "system" plugins. These types of plugins can load CSS/JS on any URL in Joomla. If you have template overrides for K2 in your template, locate and delete the file itemform.php (but only inside your template's /html/com_k2/ folder - and if it exists of course).

You're welcome :)

So it was some old leftover override. Sometimes template devs just copy the entire templates folder in K2 even if they don't use some file...