With release 6.6.6.x UW moves from using Oracle's Java SE (JRE) to using OpenJDK. This change is due to Oracle changing their licensing requirements for Java SE. As of January 2019, Oracle requires a commercial license for use of Java SE.
Due to this change, when upgrading from a previous release of UW, the custom JDBC drivers used to connect to your database servers may be deleted. The upgrade will make a backup copy of the \stoneware\jre\lib\ext folder, which holds the JDBC drivers. It will then attempt to restore any custom JDBC driver files you have added.
After upgrading, you will need to check the \stoneware\jre\lib\ext folder to verify that it still contains all of the custom JDBC driver files you added to the system. You can compare the contents to the \stoneware\backup\jre_old\lib\ext folder.
If necessary copy the custom JDBC driver files from the \stoneware\backup\jre_old\lib\ext folder to the \stoneware\jre\lib\ext folder. Only copy the ones you have added/need. Do not copy the entire contents of the folder, as it will also contain files that are incompatible with OpenJDK, as that will the cause system functionality issues.