I recently earned a badge on a few different Stack Exchange sites. My profile page of these sites now shows the "Congratulations! You've earned ..." message:
However, if click on "Track the next one", I get a popup showing all tag badges instead of the normal badges. I can't select either of the suggested badges, but since I cannot see the other ones, I cannot choose my next badge to track. This has worked correctly in the past, so something seems to have broken.
NB: The "Let us pick" button still works fine, and once a new badge is picked, the "Next badge" gear is displayed again, which can be used to select another badge from the available badges. The problem only affects the "Track the next one" button.
PS: It seems that in some cases (as reported here), the list is not populated at all. This seems to be a special case, where there are no tag badges to choose from, for example on private betas because there are no badges with 100 questions yet).