It looks your DisplayASimpleMapWmsLayerPlugin.cs just totally the same like I pasted.
And the dll you are using is 9.0.685.0.
So I think that’s maybe caused by other code in your application.
Please do a test like this for make sure that:
Download our HowDoISamples here: http://wiki.thinkgeo.com/wiki/map_suite_wms_server_edition_all_samples
Replace the dll with the latest version, then see whether that works.
If that works, replace the render logic with yours.
If that still works which means the code is caused by some other part of your project, you can compare that and find where is the different.
If the HowDoISample cannot works in your machine, please provide us more information about that, include a video which shows how to reproduce it and how the exception looks like, your OS, your .net version, your IDE etc.