If you experience DevExpress error messages like the following ones:
ASPxClientTextBox is undefined
ASPxClientTextBox ist undefiniert
__aspxCultureInfo is undefined
__aspxCultureInfo ist undefiniert
You likely forgot to correctly configure your Web.config file on your web server (IIS, IIS 6 or IIS 7).
The knowledge base article “How to manually register a HttpHandler Module” on the DevExpress website tells you how to resolve this issue.
(Keyword: IIS, IIS6, IIS 6, IIS7, IIS 7)