Generally, this error is due to having the stock version of the OS on your tablet. Shortly after the n810 was released, nokia released a new version of OS2008, so you'll need to upgrade to OS2008 version 50-2.

This involves a reflash of the tablet, though, so ensure you backup all your data in advance.

If you go to: http://www.nokia.ca/A4759013 there are instructions on how to flash up to 50-2.