From: DeskNow Knowledge Base
http://www.desknow.com/kb/

The value for Security.MaxFileUploadSize must be a number
http://www.desknow.com/kb/idx/6/090/article/
Check the MaxFileUploadSize setting in Security.cfg

It must be a valid number (and not bigger than 2 Gb).

Restart DeskNow after changing this value.