Service packs fix lots of issues in the installations but sometimes they also create problems. It becomes a pain to track down the source of the problem if you have a huge setup. A user yesterday reported the following problem after he applied SP2 on the server running InfoPath:
The format of the Canary has been tampered with for form xxxxxx by the end user.
Exception message: The security validation for this form is invalid.
Some users also reported this problem when they installed SP1. Exact cause is not known but following are some hints that can resolve the issue (at least temporarily unless you get a reliable fix):
1. Replace Microsoft.Office.InfoPath.Server.dll with the version that was working before you installed the SP.
2. Replace core.js with a newer version (worked for the user who reported this). Core.js is located in SystemDrive:\Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\LAYOUTS\INC\
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