RW 8.6.2 Can't publish or download update to higher version

Hi folks. I’ve managed to publish from 8.6.2 before, but haven’t published anything new for a few months. Tried today ‘republish all files’ several times but I just get an empty ‘Exporting your files’ pane for minutes on end with nothing happening.
And I’m getting a ‘new version (8.8.3)’ reminder every time I reopen RW, but when I try and DL it it cuts out completely after c. 20 Mb and vanishes. Curiously the ‘Check for updates’ line under Rapidweaver 8 in my Menu bar is greyed out.
Any help much appreciated.

You can download all RW versions including the latest here:

https://www.realmacsoftware.com/rapidweaver/releasenotes/