In Opensolr, a default new index, will feature a search.xml Solr configuration file (referenced further in solrconfig.xml), that you will have to address, since that contains a default /select requestHandler defined there, that may not be what you are looking for, when you are using your own custom datasets, etc.
You can fix this by simply removing the reference to search.xml inside your solrconfig.xml file:
Check out the