Adobe Reader update (11.0.01), plugin getting HTTP Error 400. The request is badly formed.

After the new update, our clients using versions 11 (11.0.01), 10 and 9 started experiencing errors on FDF based AcroForms and .Net C#.

When the pdf loads in the browser and calls the c# page from page open action, the FIRST TIME, it gives "HTTP Error 400. The request is badly formed." error (if you turn off the "Show friendly error message" option in IE), otherwise, it gives a generic error.

But after the first time, after PDF is cached, the second time onwards, it will show no error. But if you clear your cache or got to a different PDF, you will still get the same error the first time.

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Update : After doing some research, found that whenever the error occurs, in the HTTP request, come additional characters were getting inserted in the "Referer" Url and also in the FDF.

For instance, something like below (this is just a sample url and the last part is not just /A, but some special character before that too, but I couldnt paste it here.)

Referer: https://website.com/PDF/Template-p001.pdf/ A

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Anyone has any thoughts on this or seen this? Any help to find the cause and/or fix to the issues is welcome.