[xwiki-users] Problems wt Update 7.4.5 -> 8.4.4

Hello, I'm trying to upgrade our XWIKI Installation from 7.4.5 to 8.4.4 (running on an Tomcat7/Ubuntu 16.0.4 ). I folllowed the upgrade guide which basically says nothing except apt-get update and apt-get upgrade. The database should be upgraded by the distribution wizard. The apt-get commands are executed without errors. But this leaves a broken system.It failes to initialize the Solr embedded server: "Could not load conf for core xwiki: Error loading solr config from /var/lib/xwiki/data/solr/xwiki/conf/solrconfig.xml" But as far as I understood this file in untouched by the upgrade so it should be the same. So I assume the error is elseware. I double checked all other configuration files (xwiki.cfg, xwiki.properties, webx.ml, catalina.properties) they should be okay. Any help is welcome, Wolfgang