Weird behaviour after April 2015 SP 2010 CU: unauthorized exception for anonymous users on any postback at sites.asmx

Hi

This issue has been occurring on SharePoint applications after we patched our systems from SP 2010 initial version to SP2 and then with April 2015 CU.

On any postback like clicking button or link, for ANONYMOUS users only, we get user credential prompt. But cancelling the pop up we can smoothly continue the application's flow.

On using Fiddler, I can see the sites.asmx is being accessed and there it through HTTP  401 Unauthorized issue.

We have tried upgrading to Sep 2015 but no difference.

If Web security validation is turned off, the pop up disappears but that might lead to security issues.

No one on web seems to have encountered this issue.

Any help much appreciated.

thanks

smrithi