when i try to start it, it says that it can not start because steam_api.dll is missing. that error appears twice, before it finally starts. the executables is listed as original war launcher:owar.exe and original war(opengl): owarOGL_SGUI.exe. once i have clicked on ok on those error messages however, the game starts.

Deleting or Renaming OWSteam.dll should fix the issue. (or upgrading to newer version)