LookupHandle: handle out of range

I am running LN Client 6.5 on a Windows Tablet PC. Whenever a user clicks on Send, Reply, Forward, etc they recieve a red box telling that:

LookupHandle: handle out of range

The Client then shuts down.

What is causing this? I was told that it could be the OS in conflict with Client and to update 6.5 to 6.5.5, but I cannot find the link on this website to download the updates/fixes.

Help.