Inbox error after upgrade to Thunderbird 5

I upgraded to TB5 two days ago. I use it on two machines - one running Windows XP and one running Windows 7 (64-bit). I collect mail from a number of accounts which all go to the global Inbox on Local Folders. Generally speaking that Inbox folder remains empty because filters move incoming messages to other folders (which also exist within Local Folders). This arrangement worked without issue on the previous version of TB.

It runs fine on the Windows XP machine. But when I transfer to the Windows 7 machine, the first time I collect mail, I get an error saying that there was an error truncating the Inbox file after moving a message and that I should shutdown TB, delete Inbox.msf and restart it.

Doing that restores normal operation (though any messages which were moved are then also in the Inbox and have to be deleted manually).

Subsequent mail collections - and associated moving of messages by the filters - work fine.

Has anyone else encountered this?

Another peculiarity (though not a show-stopper) is that on the Windows 7 machine the window layout has thin borders between the panes, and the toolbar icons are a new style, whereas on the XP machine the layout still looks like TB3 with the thicker borders and old toolbar icons. I don't really understand why this should be different between the two machines, unless this version is not truly portable and is leaving registry entries behind.

Regards

Nick