back to blank page when saving products

[android2k]

hi everybody, here's my problem: a couple of weeks ago I put on-line the final version of VM.
Everything's running fine, apart of a strange behaviour that I investigated without success.
When I insert a new product, or when I update an existing one, at the moment I click on the "save & close" button VM leads me to a blank page. refreshing the page brings me to the joomla admin site manager and I have to re-enter the VM component where I find the new product or the modified one correctly registered.
It happens ONLY with products, neither with categories nor with any other "save" VM occurrence.
I checked obviously permissions, sessions, coockies and any other usually suggested config error.
The clone of the site on my local develop server works perfectly, though I think it could be time related, even if the remote server appears to respond in real time, and more than this I have no problem with all other functions.
I hope someone of you already faced something like this, not so severe but annoying when I have to manage many products, it's a big loss of time.
Many thanks for your time...

Further investigation lead me to this error:
PHP Notice:  Undefined index:  plugin_param in C:\sitiweb\www.smokereco.com\administrator\components\com_virtuemart\models\custom.php on line 216
I need help in order to find out what does it mean and how to solve it if possible...