You can send a message through a different route.
www.ebay.com/usr/username goes here
Top, right hand side of the page is a "contact" user tab.
Try this method. Btw, don't select you're contacting user about an item. Instead, select, "this is not about an item". The system will take you to desktop messaging system.
Some caveats: if you're using a smartphone and have the Ebay app and "open supported links in app" is checked off, you'll need to press and hold the "continue" tab, open in desktop mode will now be selectable. The alternative is to open account settings on phone, select apps, Ebay, and deselect "open supported links in app".
I know this method is a seemingly convuluted way to message your buyer, but it really isn't and it works.
