We were able to locate the exact issue after some more digging. We added a query to another collection that runs alongside the job we were executing and we were missing the collection reference in the URL. If the below query is run by itself in at least Solr 7, the error will be reproduced. http://localhost:8983/solr//select?q=*:*
Since the collection was left empty, collectionsList in HttpSolrCall.java was being set to an immutable Collections.emptyList() by the resolveCollectionListOrAlias method. Then, when collectionsList.add(collectionName) was called in the getRemotCoreUrl method the error we are seeing is thrown as it is trying to add to an immutable list. -- Sent from: http://lucene.472066.n3.nabble.com/Solr-User-f472068.html
