This is an oddball situation that might just help anyone else who encounters a similar error. Rather than use the public web server I decided I'd do most of the testing on a local WAMP sever. I cloned both Joomla and Moodle from the live site, both of which had Joomdle fully working.
I have made changes in Joomla --> Joomdle: configuration and the residual error under "System Check" was "Source IP not allowed nn.nn.nn.nn". Then I remembered I hadn't configured Moodle --> Plugins --> Manage Authentication --> Joomdle: Settings --> Joomla URL. I changed this to the local server URL and the error changed to: "Source IP not allowed: fe80::c50f:6d93:1d0b:99a6" which looks suspiciously like an IPv6 address block.
The resultion for this error is EXACTLY that same as for IPv4 (there are loads of examples for this error in the forum: 127.0.0.1; 192.168.1.1; etc...) which is to paste the IPv6 address into "Internal IP Address" in Joomdle --> Configuration --> General config.
The second issue is a question: In Joomla --> Joomdle Configuration: Connection method, the "Connection method" chosen is cURL. (cURL is enable on both the public site and WAMP). In Moodle --> Plugins --> Manage Authentication --> Joomdle: Settings: Connection method the setting is "file_get_contents" If I change this to cURL (so that both connection methods are identical) the result is the same: the System Check shows all green ticks.
I assume "file_get_contents" is a fallback for when cURL is not available? Which connection method is preferable - or doesn't it matter?
