problem authenticating your post information - Error in fresh install

problem authenticating your post information - Error in fresh install
when i access "/ajax/switchextension.php" of Vanilla install, browser shows "problem authenticating your post information" error.
This is a fresh install, how to correct this. I cant post any discussions, enable extensions, all these give the same error.
TIA..
when i access "/ajax/switchextension.php" of Vanilla install, browser shows "problem authenticating your post information" error.
This is a fresh install, how to correct this. I cant post any discussions, enable extensions, all these give the same error.
TIA..
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