I think this has something to do with 0.11.6.
I made no changes to my network configuration.
I uninstalled Obsidian, renamed the Obsidian folder in C:\Users\NAME\AppData\Local\Obsidian\Obsidian.exe and then reinstalled Obsidian v 0.11.3.
I was able to browse community plugins as normal as well as install plugins and then check for updates.
I uninstalled Obsidian, deleted the new Obsidian folder in C:\Users\NAME\AppData\Local\Obsidian\Obsidian.exe and changed the old folder back.
On installing v 0.11.6, the problem as described above returned.