Source: WowAce

When Rarity pops up the achievement-like panel like in the first image shown in the Images category, and I click it, the achievements panel opens with somewhat garbled content along with an error:

Message: ...ns\Blizzard_AchievementUI\Blizzard_AchievementUI.lua:2544: bad argument #1 to 'band' (number expected, got nil)
Time: 03/24/18 19:12:13
Count: 2
Stack: ...ns\Blizzard_AchievementUI\Blizzard_AchievementUI.lua:2544: bad argument #1 to 'band' (number expected, got nil)
[C]: ?
[C]: in function `band'
...ns\Blizzard_AchievementUI\Blizzard_AchievementUI.lua:2544: in function `AchievementFrame_SelectAchievement'
Interface\FrameXML\AlertFrameSystems.lua:421: in function <Interface\FrameXML\AlertFrameSystems.lua:401>

Clicking on actual achievement pop-ups work without errors.
I THINK this started to happen when I installed Pawn, but I can't be sure. I have reported this to the author of Pawn, but as Pawn doesn't interact at all with the achievement window, it doesn't seem likely to be a fault in Pawn. But who am I to tell. :)

Putting this on hold, as the functionality is deeply entrenched within the Blizzard UI and I haven't found a good way to work around it. It's probably not worth the effort at this time.

Guess it was much simpler than I had originally thought. Oh, well!