You should contact support.
These are quotes from the Steam FAQ:
2. Help, I get this error: “Couldn’t load plug-in”!
If you get this error, the game was unable to run your Google Chrome Flash plugin. Just open the options menu (on the right side of the The Settlers Online start menu) and change the plugin setting to “Adobe Flash Player plugin”.
7. I have a Chrome/Flash player plugin error message at launch. What can I do?
You can fix the issue “the hard” way by following the steps:
1.) Make sure the game is not running
2.) Go into the “The Settlers Online” game folder.
3.) Create a folder “plugins”.
4.) Copy the file “pepflashplayer.dll” from your Google Chrome into this new folder.
5.) Edit the file “package.json” in your “The Settlers Online” game folder and change the path to the plugin to: 'plugins\\pepflashplayer.dll'
6.) If there is no path: please add the following to the list of “chromium-args”: --ppapi-flash-path='plugins\\pepflashplayer.dll' --ppapi-flash-version=18.0.0.233
7.) The Flash Version number (18.0.0.233) depends on which Flash version you have installed along with Google Chrome.