Nevermind - I had a trailing / after the URL in admin/includes/configure.php. This caused the URL of the ADD action to add another /, which caused a new session instance each time the attribute inventory was modified. It all works perfectly now!
If anybody needs help with this addon, let me know.
I don;t know if you're still active on this forum but I hope so!
I have just installed the QTPro 4.6.1 contribution and have exactly the same problem as you mentioned here. However, if I remove trailing /s from the URL's in my admin/configure.php file then other things don't work properly e.g. the display is misarranged or the page isn't found (because there isn't a / where there should be in the address) or the main admin menu disappears altogether.
Can you tell me which URL in your admin/configure.php file was causing you the problem? I realise it is a long time ago now but thought I would try asking anyway!
Thank you in advance for any help you can give.