On my first attempt at neck sizing my 6.5x55, 20% of the necks split.
Never had such a problem with the 9.3x62 when neck sizing.
Using a Redding die, RWS cases, fired only about 3 times. Not using hot loads.
Any ideas?

The neck area in the chamber may be larger than what it should be. This may overwork the brass when sizing down. Work hardened brass may need annealing. It could be just the brass, try a different brand/make.

Bad lot of brass?

I thought that brass doesnt get much better quality than RWS. These were expensive-to-acquire post factory load cases. Will try some Winchester brass on hand.
Do you think that the other 80% of cases will be OK to load and safely use, i.e. can I continue to use these until they split or otherwise expire?

I use my brass until I get split necks and have had no problems. As 243win said, try annealing.

Keep using the brass till it splits. No danger involved.

have you annealed it?

are you lubing the inside of the necks?
I use a bore brush and some spray lube on it and run it down the neck before I resize a case, whether necksizing, partial resizing or fully length resizing..