In some cases SynAdmin shows the following message in "Full-Text Search" Tab: "FTS-Index is locked. Please delete the file 'write.lock' in the directory 'solr/data/index' of the FTS-Index and restart Tomcat."
Following the instructions the file "<Tomcat install directory>/webapps_custom/FTS-Index/solr/data/index/write.lock" has to be deleted and the FTS-index application restarted.

SynAdmin - the FTS-index is locked