After spending sometimes on Alfresco configuration, I've noticed that the error value seems to be related to Solr search service. It has teh similar number in the section for "Index Lag (Seconds):" and the number was preceded by "Unavailable: ".
I also noticed that Solr was set as Search Service in the Search Manager, so I switched it to Lucene. Then the query started to work.
Since I'm new to Alfresco and the environment I'm using was already there, I'm not sure if all the configurations are done corresctly. As far as Solr goes, probably it's not configured appropriately, I guess.
But even though now I can use query, I'm still not sure about the error and what I should do to be able to use Solr. Besides acquiring more knowledge on it, can anyone still point out the cause of the error initially I had?
Thank you