The problem also shows up in in-game dynasty editor from Crusader Kings II: Customization Pack DLC.
String with identifier coa_plain_pagan is missing from localization files in game.
Code:
grep -i coa_plain *
RulerDesigner.csv:coa_plain;Plain;Plain;Schlicht;;Liso;;;;;;;;;x
SwordOfIslam.csv:coa_plain_muslim;Plain;Pur;Einfach;;Simple;;;;;;;;;x;
Edit: Fixed in beta patch 2.1.5.5.