Yes, thank you! It was a temporary problem with the extensions catalogue according to Plesk support. Looks like there are others that have had similar problems but those issues have been set as resolved with no explanation. Hope this can help others in the future, if/when this happens again.
I get the message "Internal server error" under the Extensions Catalog tab and the message "GraphQL error: Internal server error" when I try to check for new extension updates. Installed extensions are still shown under My Extensions tab.
The same message is shown on several of our VPS servers.