When I post the spoiler tag, it is ignored. Within a post it's just not visible, when posted on its own I get the error message "Please enter a valid message."
Maybe you need to make sure you aren't getting any error codes in the JS console for the browser - or stop any ad-blockers or temporarily disable antivirus.
The mistake I made was clicking the spoiler link in the editor thinking that I could add the content that way in the overlay, but it only asks for a title. It's designed to use on existing text.