After installing Exchange 2010 SP2 I started to receive errors in OWA:
Your request couldn't be completed.
This may have occurred for security reasons or because your session timed out.
This error does not happen if you open OWA via HTTPS.
To be able to use OWA via HTTP without this error you need to edit web.config file located in:
C:\Program Files\Microsoft\Exchange Server\V14\ClientAccess\Owa\web.config
and change requireSSL to false
Save the file and OWA should work fine.