Why am I getting ‘cannot access outlook.ost’ issues with Office 365 Lync?

We use O365 to provide Lync messaging. So when I rebuilt my PC I thought I needed to re-install the client; so I logged into the O365 web site and selected the install option. Turns out this was a mistake. I had Office 2013 installed, so I already had the client, I just had not noticed.

If you do install O365 Lync client (as well as Office 2013 one) you get file access errors reported with your outlook.ost files. If this occurs, just un-install the O376 client and use the one in Office 2013, the errors go away