Sometimes your Xcode project file gets messed up, especially if you have an old project and first created it with an older version of Xcode/iPhone SDK.
What you need to do is open up the project file in a text editor, search for the 'long string' from your error and manually erase that line.
In fact, you should just go ahead and erase any line that points to any provisioning profiles. Then reopen the project in Xcode, go to the settings and reselect your new profile. This clears up issues like that most of the time.
The lines that point to the provisioning profiles will look like this:
PROVISIONING_PROFILE = "487F3EAC-05FB-4A2A-9EA0-31F1F35760EB";
"PROVISIONING_PROFILE[sdk=iphoneos*]" = "487F3EAC-05FB-4A2A-9EA0-31F1F35760EB";
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