Update issues with 2.3.3-Release

[blimpyboy]

After configuring OpenVPN server earlier today I tried to carry out the final step of installing the client export package.  This failed, but I now see I have bigger problems as the Dashboard is telling me it is 'Unable to check for updates'.  I noticed that jimp had a sticky thread with a related issue, but that appears to be for versions prior to 2.3.2 - I'm using 2.3.3-Release.

I then tried to update from an SSH session using both option 13 (Update from console) and 'pfSesne-upgrade -d' from a shell.  Both of those fail with loads of 'Certificate verification failed for /C=SE/O=AddTrust …' errors.  A 'pkg update' gives similar errors.

I should state that Internet access is just fine and the network is showing no other issues whatsoever. 
No idea what's happening here so would be grateful for any pointers.