Many thanks, Kenneth.
OK. First, I reflashed the Core with tinker (spark flash --usb tinker), then I reinstalled spark-cli using sudo npm install -g spark-cli@0.4.94 as you said - took forever, but no drama. Then set the config to the local cloud. Running spark setup failed the first time - i got the same errors as before:
/usr/local/lib/node_modules/spark-cli/commands/SerialCommand.js:136
for (var i = 0; i < ports.length; i++) {
^
TypeError: Cannot read property 'length' of undefined
etc
etc
I then restarted the local cloud and tried spark setup again. This time it created the account, and when I rebooted the local cloud server again it recognised the account and the core is breathing cyan. But I am still getting the decryption errors all the time on the local cloud server terminal output - about every 25.030 - 25.040 seconds (!). Each time the Core flashes cyan and then carries on breathing.
I then try “spark list” and it names the core, but says it’s offline - even though the server output says that the core is online!
When I try a few “curl” commands they don’t even go through - message comes back saying that it cannot find the server even though the URL’s are the same. I gave up.
Back to the PC. Trying npm install -g spark-cli@0.4.94 was a total disaster, so I reinstalled spark-cli. It works fine on the Spark cloud. However, when I fire up the local server there is no way I can get to set up an account - does not give me any token etc. The Core behaves the same way as with the RPi: breathing and then intermittent flashing with these messages:
CryptoStream transform error Error: error:06065064:digital envelope routines:EVP
_DecryptFinal_ex:bad decrypt
onSocketData called, but no data sent.
1: Core disconnected: socket close false { coreID: '55ff6d065075555351161787',
cache_key: '_10',
duration: 25.085 }
Session ended for _10
Connection from: ::ffff:192.168.1.6, connId: 12
CryptoStream transform error TypeError: Cannot read property 'length' of null
CryptoStream transform error TypeError: Cannot read property 'length' of null
on ready { coreID: '55ff6d065075555351161787',
p: '::ffff:192.168.1.6',
product_id: 0,
firmware_version: 11,
cache_key: '_11' }
Core online!
CryptoStream transform error Error: error:06065064:digital envelope routines:EVP
_DecryptFinal_ex:bad decrypt
The “ffff:192.168.1.6” looks a bit funny?
Since I cannot get an account, I cannot log in so I cannot try any “curl” commands to test it.
It used to work… 
Any ideas, gratefully received.