Hi Andrea,
was it yourdomainname.com/folder/ and is now yourdomainname.com?
Yes
The structure uses subdirectories, and the only upgrade I’ve made is the most recent version of WP – but that was a while ago, and the site was working fine after that…
Did you try disabling plugins?
Yes – but this doesn’t help with the adding of new sites. And when I enabled them again and tried to upgrade automatically, while it says the upgrade has completed fine, when I browse away from the page, I get an alert to upgrade again…now there are 3 plugins waiting fro upgrade…