Datasource Definition is:
<Resource name="jdbc/PostgresDB" auth="Container" type="javax.sql.DataSource" 
maxActive="100" maxIdle="0" maxWait="10000" username="xxx" password="xxx" 
driverClassName="org.postgresql.Driver" url="jdbc:postgresql://localhost:5432/
dbname"/>

I put the driver into common/lib and into webapps/.../WEB-INF/lib.

happens with 5.5.12 and 5.5.15 beta.

stacktrace is:
org.apache.tomcat.dbcp.dbcp.SQLNestedException: Cannot create JDBC driver of 
class '' for connect URL 'null'
        at org.apache.tomcat.dbcp.dbcp.BasicDataSource.
createDataSource(BasicDataSource.java:780)
        at org.apache.tomcat.dbcp.dbcp.BasicDataSource.
getConnection(BasicDataSource.java:540)
        at dti.common.logic.impl.ConnectionPool.
getConnectionInternal(ConnectionPool.java:106)
        at dti.common.logic.impl.ConnectionPool.getConnection(ConnectionPool.
java:34)
        at dti.common.logic.impl.UserManagerWrapper.doLogin(UserManagerWrapper.
java:21)
        at dti.common.actions.LoginAction$DoList.process(LoginAction.java:41)
        at dti.common.actions.PasAction.processRequest(PasAction.java:153)
        at dti.common.actions.PasAction.execute(PasAction.java:58)
        at org.apache.struts.action.RequestProcessor.
processActionPerform(RequestProcessor.java:421)
        at org.apache.struts.action.RequestProcessor.process(RequestProcessor.
java:226)
        at org.apache.struts.action.ActionServlet.process(ActionServlet.java:
1164)
        at dti.common.control.PasActionServlet.process(PasActionServlet.java:98)
        at org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:415)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:709)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
        at org.apache.catalina.core.ApplicationFilterChain.
internalDoFilter(ApplicationFilterChain.java:252)
        at org.apache.catalina.core.ApplicationFilterChain.
doFilter(ApplicationFilterChain.java:173)
        at org.apache.catalina.core.StandardWrapperValve.
invoke(StandardWrapperValve.java:213)
        at org.apache.catalina.core.StandardContextValve.
invoke(StandardContextValve.java:178)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.
java:126)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.
java:105)
        at org.apache.catalina.core.StandardEngineValve.
invoke(StandardEngineValve.java:107)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.
java:148)
        at org.apache.coyote.http11.Http11Processor.process(Http11Processor.
java:869)
        at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.
processConnection(Http11BaseProtocol.java:667)
        at org.apache.tomcat.util.net.PoolTcpEndpoint.
processSocket(PoolTcpEndpoint.java:527)
        at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.
runIt(LeaderFollowerWorkerThread.java:80)
        at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.
run(ThreadPool.java:684)
        at java.lang.Thread.run(Thread.java:595)
Caused by: java.sql.SQLException: No suitable driver
        at java.sql.DriverManager.getDriver(DriverManager.java:243)
        at org.apache.tomcat.dbcp.dbcp.BasicDataSource.
createDataSource(BasicDataSource.java:773)
        ... 28 more