On some sites, when using the review queues, the button which is disabled (because it's not compatible with your (lack of) actions so far) has become invisible:
For an example of how it did (and should) look like, this is the situation on Stack Overflow:
I originally posted the bug here but it seems to be an issue on more sites. It could be that the new site theme will fix things; for a similar problem on Ask Different I posted a bug report here and it has been fixed. That specific problem still affects The Workplace.