Created Unassigned: AutoCompleteExtender with pagemethod [28024]

Hi,

I've got an issue with the AutoCompleteExtender when not specifing ServicePath i.e. using PageMethods instead. This is an old site converted from ASP.NET 3.5 to 4.0. If I create a ASMX webservice and point it to the service instead it works. I have also tried to create a separate project to isolate the problem but there it works both with and without ServicePath specified. The output window in VS is full of error like this:

```
Exception was thrown at line 249, column 3 in eval code
0x800a1391 - JavaScript runtime error: 'O' is undefined
Exception was thrown at line 3431, column 10 in http://localhost/mytestsite/ScriptResource.axd?d=61nxzTAOMpUjuhdR4zSdPKbPg7XNdeQB2iOw7TJrnJfuKs0ZGvdxsbjscmD5aJnkNJybYApTiXXX--lDhO0Ws6qqmY093u4BfnJ2MOmuW-FDn3vwSIDvZrInxJRX1mwskcFuQHN7CDGY53IK1q5ozCEycR0rSfJZ2_2VV0Y6V2WcKYD-bBxnFtgw2qgewkbJ0&t=ffffffff82481a10
0x800a139e - JavaScript runtime error: Sys.ArgumentException: Cannot deserialize. The data does not correspond to valid JSON.
Parameter name: data
Exception was thrown at line 329, column 3 in eval code
0x800a1391 - JavaScript runtime error: 'v' is undefined
Exception was thrown at line 3431, column 10 in http://localhost/mytestsite/ScriptResource.axd?d=61nxzTAOMpUjuhdR4zSdPKbPg7XNdeQB2iOw7TJrnJfuKs0ZGvdxsbjscmD5aJnkNJybYApTiXXX--lDhO0Ws6qqmY093u4BfnJ2MOmuW-FDn3vwSIDvZrInxJRX1mwskcFuQHN7CDGY53IK1q5ozCEycR0rSfJZ2_2VV0Y6V2WcKYD-bBxnFtgw2qgewkbJ0&t=ffffffff82481a10
0x800a139e - JavaScript runtime error: Sys.ArgumentException: Cannot deserialize. The data does not correspond to valid JSON.
Parameter name: data
```
Also when looking at the response in the browsers developer tools the return seems to be the whole html page instead of something like {"d":["abcNto","abcYlq","abcCbv","abcGoi","abcWyn","abcCsn","abcLrh","abcNom","abcWkb","abcVbt"]}

Any clues where to start looking?