vmError: You are not an administrator... when adding products

[dospuntos]

After successfully adding a couple of new products in Virtuemart, I suddenly get this error message when saving:

vmError: You are not an administrator or the correct vendor, storing of product cancelled

When I tried with another browser (IE instead of Chrome), it worked again for a while, and suddenly I got the same message.
Any ideas what this is all about?
I'm using Joomla 2.5.14 and VirtueMart 2.0.22b

[jenkinhill]

Try increasing the Session Lifetime in Joomla Gobal Config/System/ Session Settings

Also see http://forum.virtuemart.net/index.php?topic=99353.0