and the browser appears to 'lock up'
Where your On Premise environment uses Active Directory authentication on connections to the application server, Perfectforms has an alternative upload mechanism that allows uploads to the server in such an environment.
It is found in the Administer/Account/Settings under the “Misc “ section
‘Use Alternative Upload Mechanism.’
If you check and enable that, you should then be OK.
Please note that the client browser settings should allow the javascript pop-up that this alternate mechanism uses.