I use H2 in DB2 mode for tests.

I am getting the next value for a sequence with:
SELECT TEST_SEQUENCE.NEXTVAL FROM SYSIBM.SYSDUMMY1

It worked fine for both DB2 (LUW) and H2 (in DB2 mode).
After I upgraded H2 to 2.1.212, it stopped working.

http://www.h2database.com/html/migration-to-v2.html says that
Undocumented Oracle-style .NEXTVAL and .CURRVAL expressions are restricted to Oracle compatibility mode.

However, this expression should also work in DB2 mode because it works on real DB2.