Causes of getting a java.lang.VerifyError

I'm investigating the following java.lang.VerifyError:

java.lang.VerifyError: (class: be/post/ehr/wfm/application/serviceorganization/report/DisplayReportServlet, method: getMonthData signature: (IILjava/util/Collection;Ljava/util/Collection;Ljava/util/HashMap;Ljava/util/Collection;Ljava/util/Locale;Lorg/apache/struts/util/MessageRe˜̴Mt̴MÚw€mçw€mp:”MŒŒ
                at java.lang.Class.getDeclaredConstructors0(Native Method)
                at java.lang.Class.privateGetDeclaredConstructors(Class.java:2357)
                at java.lang.Class.getConstructor0(Class.java:2671)

It occurs when the jboss server in which the servlet is deployed is started. It is compiled with jdk-1.5.0_11 and I tried to recompile it with jdk-1.5.0_15 without succes. That is the compilation runs fine but when deployed, the java.lang.VerifyError occurs.

When I changed the method name and got the following error:

java.lang.VerifyError: (class: be/post/ehr/wfm/application/serviceorganization/report/DisplayReportServlet, method: getMD signature: (IILjava/util/Collection;Lj    ava/util/Collection;Ljava/util/HashMap;Ljava/util/Collection;Ljava/util/Locale;Lorg/apache/struts/util/MessageResources ØÅN|ØÅNÚw€mçw€mX#ÖM|XÔM
            at java.lang.Class.getDeclaredConstructors0(Native Method)
            at java.lang.Class.privateGetDeclaredConstructors(Class.java:2357
            at java.lang.Class.getConstructor0(Class.java:2671)
            at java.lang.Class.newInstance0(Class.java:321)
            at java.lang.Class.newInstance(Class.java:303)

You can see that more of the method signature is shown.

The actual method signature is:

  private PgasePdfTable getMonthData(int month, int year, Collection dayTypes,
                          Collection calendarDays,
                          HashMap bcSpecialDays,
                          Collection activityPeriods,