This has been found in DSpace 5.9, but probably also affects 6.3 with JDK 7, as well as 7.x if JDK 7 is still an option.
DS-3854, PostgreSQL JDBC driver has been updated. However, the version registered in pom.xml downloads a Java8 (JDBC 4.2) compatible PostgreSQL JDBC driver, which leads to the exception below:
Exception: org/postgresql/Driver : Unsupported major.minor version 52.0
java.lang.UnsupportedClassVersionError: org/postgresql/Driver : Unsupported major.minor version 52.0
Changing the postgres version to 42.2.1.jre7 in pom.xml solves the problem: