Outlook 2007 PST - Cannot expand the folder. The set of folders cannot be opened.

Hello, a user is having difficulty opening a PST file when working remotely over VPN. This is on a Windows 7 x86 workstation using Outlook 2007. The PST file is located on a network share, but this is automatically mapped (and is accessible) before Outlook is opened on both the corporate network and VPN.

When physically connected to our corporate network, Outlook opens fine and displays/expands the PST file as normal. Repeating the same excercise over VPN, the PST is listed but gives an error when we try to expand -

'Cannot expand the folder. The set of folders cannot be opened. The file ***PST file path*** cannot be opened.'

When we try to open the data file (File > Open > Outlook Data File), we can see the PST in the network share, but receive the following error -

"***PST filename*** This file is currently not available for use on this computer.'

Thanks