Was going through old emails (Version 9.1 (3096.5)) and dragging emails to the sorter. Got to 49 in the inbox and the next email that I dragged popped up with this incredibly useful error message
*** -[__NSPlaceholderDictionaryinitWithObjects:forKeys:count:]: attempt to insert nil object from objects[0]
The email did, however, appear in the inbox
Version is DTPO 2.8.7, OSX 10.11.1
Edit: Looks like it is only one specific email causing the problem