Resolving InfoPath 5566 Error

If you get a 5566 error while calling a SharePoint web service through a web based form make sure you have a look at the following.

• You call the web service at a site or site collection level (eg: h ttp://myserver/mysitecollection/_vti_bin/UserProfileService.asmx) 
• The user who is opening the form has access to the subject site collection
• Anonymous access is disabled through Central Admin
• In the web applications web.config file you have Proxy AutoDetect = false (on many cases I could leave this the default value true without a problem)
• If you have multiple web front ends, depending on your configuration you might have to add an entry to the hosts file (Eg: MyMOSS 127.0.0.1) so that InfoPath Form services calls the web service on the same server avoiding NLB..
• If you made the above change be sure that the address you add (MyMOSS) is the same name that you assign in IIS for that application. Basically it should be the same address clients are using.

Before applying the above steps you can have a look at the IIS logs. Probably you will see requests with error code 401 and the user who requests that page will be missing.. After you resolve the issue the user name requesting the page should be logged at each call.