There is a two-way communication between the Alfresco server and the Solr server. Alfresco send http request to solr and search for content,solr call alfresco's restful api to update the indexes.
Your exception occurs when solr call alfresco webscript api to track model changes,it seems that there was a connection error between solr and alfresco.
As the reason why you can search any folder with admin user maybe that the operation that you do using admin user just only need to access database(not access solr).