Older Mac Mini with network gremlins? - cannot run software update, but has working ethernet connection

I have an older Mac Mini that I'm trying to set up to run a VPN service: Mac Mini (2009) Intel Core 2 duo OSX 10.5.8

Ethernet connected via a cat5 cable, AirPort is off - connection seems robust:

• After turning on sharing, can ssh to the machine
• From shell (or ssh) can ping/curl to every site I've tried
• Running Network diagnostics says the internet connection seems fine
• From network utilities can run dig/whois/traceroute/etc

However, from some applications the internet seems 'broken':

• Running SoftwareUpdate times out with -1001 error, asks me to check my internet connection
• Both Safari and Firefox (yes, that old) refuse to open any sites. 'Server dropped connection' is sometimes given as the cause. I understand that many sites will refuse to let old browsers connect, but a) not all of them and b) most will send a page saying 'hey, your browser is too old'

Network settings seem OK:

• No proxies are set
• DNS servers (from router) are both reachable via ping

Misc. stuff that I've noticed, may not be pertinent:

• Downloaded an openVPN DMG, mounted, but cannot install - package path invalid, but it does exist. GUI installation also fails
• Apple ID associated with the computer now belongs to my daughter - but was never challenged for password
• On initial boot system clock was 'before 2000'; but when I went to set manually it was already connected to an ntp server and time/date was correct

Thanks for any insight into this issue