What version of the game do you have? Latest is 1.0.124 (shown on the main menu, below the column of buttons).
Try saving the game, switching to a harder difficulty level, load the game and switch back. That helped some people with the GOG version who ran into this error in an earlier game version.
If that doesn't help, email supportdc@larian.com, with a description of the problem and the report.zip file generated by the
Dragon Commander support tool.
