Error: There was an error received from the Ektron Solr Search Admin service while attempting to retrieve the status of the Solr Core corresponding to your CMS site.
There was an error reading crawl flags from the CMS database corresponding to Core xxxxxx . Specified cast is not valid.
Solr Search is not backwards compatible.
Your Solr search version must be the same as or higher than your CMS version. So 9.1 Solr works with a 9.1 or a 9.0 CMS. But vice-versa is not recommended.