WP failes to delete multiple post categories with bulk-delete option

I installed a new theme and imported it’s demo content. Subsequently, I wanted to delete the superflous post categories using the bulk option to delete them. I get a message “Do you really want to do this?” but with out an “OK” or “Cancel” button. Is there a setting that denies me doing this? This has noting to do wit the theme I installed, though, because the standard themes (twenty…) themes show the same error.

Thanks in advance for any help 🙂