The 1874 RLDS edition accidentally dropped the indefinite article a before serious contention. The 1908 RLDS edition restored the a. The critical text will maintain the original reading with the a here in Helaman 1:2. See under Alma 25:8 for some discussion on whether contention should take a determiner or not.
Summary: Maintain in Helaman 1:2 the original reading with the indefinite article a in “a serious contention”, the reading in 𝓞.