This happened to me as well. I had two open slots. Filled one a week or two ago with a new character, and went to make another tonight... only to find the empty slot missing, and "10 of 9 character slots in use" at the bottom of the page. Had been 10 of 11. I didn't delete a character or make any other changes, they just vanished. Thankfully, my new character did not. I've contacted support. Hopefully I get someone that's familiar with the issue.