Not a particularly hard problem to solve, but just making a note here because some of the DevExpress site appeared to be down today. Long story short: my customer was getting this error after installing a new version of our ASP.NET application. The error is because I upgraded the controls from the last build. The DevExpress controls include the version number in the name, so when they are updated you get new DLLs in your /bin folder. The install I sent to him only had the new controls, but since he installed it over a previous build it had both the old DLLs and the new ones, causing the error.
I don’t anticipate upgrading those controls all that frequently, but hopefully I’ll remember to tell him to delete the /bin folder before installing.