Further to below, I have resolved one problem (#3), but others remain.
I have been in touch with the Host, who asks what are the server requirements of the plugin?
Remaining problems are:
1. When I save edit-page I now get continuous server-error messages (reads:The website encountered an error while retrieving http://xxxxxxx.com/wordpress/wp-admin/themes.php?page=editcss. It may be down for maintenance or configured incorrectly.)
2. Some CCS revisions are listed, but only up to about 1 week ago. When I go to the comparison page, the new slider identifies many more going back 4 months, but appears to be broken. I just get message "Sorry, something went wrong. The requested comparison could not be loaded".
I am not worried about revisions of a week ago or longer, but do need revisions made in the last few days, which cannot be accessed at all now, which is a serious defect.
Other problem about delay in reading saved edits is solved.