Hello,
Please check the GAC and ensure that the above assembly is there (check the assembly name and the version, DevExpress.Web.ASPxTreeList.v9.3 and version 9.3.3.0 respectively) on each WFE server.
Then do an iisreset, clear the browser's cache and try again.
Please confirm whether you performed all web.config manual changes required by this web part to work correctly.
Thanks,