I didn't understand, whether you FIXED this or still Looking...!!
Is it for all the users or for particular, try changing the machines and perform the same.Check whether those users are able to attach the documents.
If nothing happens there, try to Re-start the services/Clean the "TEMP" (May not help) and Increase the Heap Size( it looks too low for me).
Everything is allright & Still on Issue, Log a reference with HP. (Check the pre-requisites before).
