Follow SUPPORT-10649:

Error occurs in 

info.magnolia.search.solrsearchprovider.logic.result.SingleSolrSearchResultTransformer

on the line

anAbstract = (String)solrDocument.getFieldValue("abstract");

The result of getFieldValue is in this case an ArrayList of 1 value. All my fields return an ArrayList.

Investigation shown that "abstract" field was configured to use "text_general" fieldType:

<field name="abstract" type="text_general"/>

While text_general fieldType was configured as "multiValued":

<fieldType name="text_general" class="solr.TextField" positionIncrementGap="100" multiValued="true">

Also from Javadoc of Solr function getFieldValue of a SolrDocument may return a single value or a collection of values:

Object org.apache.solr.common.SolrDocument.getFieldValue(String name)

Get the value or collection of values for a given field.

However when implementing function info.magnolia.search.solrsearchprovider.logic.result.SingleSolrSearchResultTransformer.transform(SolrDocument, Collection<?>) we casted a single String object to get its result:

String anAbstract = (String) solrDocument.getFieldValue("abstract");

This is a bug when implementing the module, please fix it accordingly. Any workaround or suggestion are welcome!