Sometimes when you index your site content, you may get the following error:

Drupal\search_api_solr\SearchApiSolrException while indexing: Solr endpoint http://solr-1234.hostedapachesolr.com:80/ internal Solr server error (0). in Drupal\search_api_solr\SolrConnector\SolrConnectorPluginBase->handleHttpException() (line 949 of /var/www/html/web/modules/contrib/search_api_solr/src/SolrConnector/SolrConnectorPluginBase.php).

If you get this error, but otherwise your site can connect to the Solr server successfully, there's a chance your Solr timeout setting needs to be increased. See this issue on Drupal.org.

To increase the timeout, edit the Search API server for Hosted Apache Solr on your Drupal site, and increase the 'Index timeout' under the solr backend settings.

Thanks to Mikell S for discovering this fix!