I run into this error quite frequently when planning to raid treasure islands. I select both an offensive and defensive unit and am then told by the game client choose both offensive and defensive units. I have to quit the game and re-launch it to get rid of the error. I cannot recreate the error at will, it just randomly happens. I don't recall it ever happening the first time I try to raid a treasure island after signing in. Sometimes I can play for hours without it happening.

Whiplash Island -85; -1864 on The Pirate Bay server

uid 1462105 / pp23037712 / The Pirate Bay / segment00

Windows 7. Client version 4.1.0 (4.1.0-1679

Snidely Whiplash - Plarium ID 23037712