Looks like that after upgrade to 3.1.1 new XUnit reporter (ut_junit_reporter) supports only suites where test case names (procedure names) are strictly lower case.
The reason seems to be that somewhere in suit builder conversion to lower case happens only partially.

My test package looks like this:

CREATE OR REPLACE PACKAGE TestPkg1
AS

-- %suite(Some desc)
-- %suitepath(de.suit1)

-- %test(My Test 1)
PROCEDURE Test1;

END TestPkg1;

I've added some debugging in the reporter type in ut_junit_reporter.print_test_elements in the very beginning:
raise_application_error(-20666,a_test.path||'/'||a_test.name);

and see the following:
de.suit1.testpkg1.Test1/test1

Subsequently the get_path fails to find a path and classname stays unfilled. And without classname the whole report becomes unusable.

If the procedure is renamed to test1 than everything works. But in 3.0.4 there was no such restriction.