Not a valid verification number

Hi Andy,

we really love this plugin and use for most of our downloads. However, the plugin fails at random? Some people manage to get the downloads, whilst quite a few others actually see the following error: not a valid verification number.

This is of course frustrating both to the end user and to us, because we’ll have to send them the download manually, which defects the whole scope of the plugin.

Can you guide us to resolving this problem?

Cheers
David