I have the same HTTP error after selecting the JQUI skin. Have been using the BlueShift skin without any problems. I'm using MPExtended 0.5.3. Only way to fix the error is to remove WebMPExtended and do a reinstall (done this a few times now). Also the jqui.dll is where it should be.
Regurge is right.... but I fear it is possible that I did not use all the proper strings for flawless translation. @Oxan might remember if I had some sting issues or not, but I have a vague recollection that I might have messed that up.
Okay, I need to update the strings. Until I do, you can change it yourself to what you want, just edit _Layout.cshtml (with any text editor) in the folder C:\Program Files\MPExtended\WebMediaPortal\www\Skins\jQui\Shared.
There will be a line, right under each call for (viewbag) in the menu. Each will be similar to the first one listed (the first one is for Movies). I have given you the example of the first one and the change you would make: