WebSphere Commerce 7 | Solr | Preprocess failed to run | SolrCore 'collection1' is not available | Could not load config file "search/solr/home/collection1/solrconfig.xml"

After migrating custom Solr files and if you try to run Solr Preprocess, sometimes you might face the following exceptions in running di-preprocess command,

INFO: Solr server core MC_10001_CatalogEntry_Unstructured_en_US status is initialized.
Aug 5, 2016 1:51:45 PM com.ibm.commerce.foundation.dataimport.process.DataImportProcessorMain executeDIH
INFO: Solr server core MC_10001_CatalogEntry_Unstructured_es_ES status is initialized.
Aug 5, 2016 1:51:45 PM null null
INFO: CWPKI0027I: Disabling default hostname verification for HTTPS URL connections.
Aug 5, 2016 1:51:46 PM com.ibm.commerce.foundation.dataimport.util.DataImportHelper printException
INFO: Data import process was unsuccessful. An unrecoverable error has occurred.
Throwable occurred: org.apache.solr.client.solrj.impl.HttpSolrServer$RemoteSolrException: Expected mime type application/octet-stream but got text/html. Error 500: {msg=SolrCore 'collection1' is not available due to init failure: Could not load config file /opt/IBM/WebSphere/CommerceServer70/instances/live/search/solr/home/collection1/solrconfig.xml,trace=org.apache.solr.common.SolrException: SolrCore 'collection1' is not available due to init failure: Could not load config file /opt/IBM/WebSphere/CommerceServer70/instances/live/search/solr/home/collection1/solrconfig.xml
        at org.apache.solr.core.CoreContainer.getCore(CoreContainer.java:827)
        at org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:317)
        at com.ibm.commerce.foundation.internal.server.services.search.servlet.solr.SolrSearchDispatchFilter.doFilter(SolrSearchDispatchFilter.java:100)
        at org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:217)
        ...........
        ...........
        at com.ibm.ws.http.channel.inbound.impl.HttpInboundLink.ready(HttpInboundLink.java:278)
        at com.ibm.ws.tcp.channel.impl.NewConnectionInitialReadCallback.sendToDiscriminators(NewConnectionInitialReadCallback.java:214)
        at com.ibm.ws.tcp.channel.impl.NewConnectionInitialReadCallback.complete(NewConnectionInitialReadCallback.java:113)
        at com.ibm.ws.tcp.channel.impl.AioReadCompletionListener.futureCompleted(AioReadCompletionListener.java:175)
        at com.ibm.io.async.AbstractAsyncFuture.invokeCallback(AbstractAsyncFuture.java:217)
        at com.ibm.io.async.AsyncChannelFuture$1.run(AsyncChannelFuture.java:205)
        at com.ibm.ws.util.ThreadPool$Worker.run(ThreadPool.java:1662)
Caused by: org.apache.solr.common.SolrException: Could not load config file /opt/IBM/WebSphere/CommerceServer70/instances/live/search/solr/home/collection1/solrconfig.xml
        at org.apache.solr.core.CoreContainer.createFromLocal(CoreContainer.java:530)
        at org.apache.solr.core.CoreContainer.create(CoreContainer.java:597)

Ideally, there won't be any "collections1" folder in the WebSphere Commerce Solr files. This might have happened because couple of Solr configuration files are missed in Search folder of the WebSphere Commerce installation directory.

Check whether all the Seach configurations files are in place and it should resolve your issue.