Bad choise. Is there a way to move the data to other machine and reinstall everything with ext4 fs, no raid? I ran 7 DS220+ and 1 DS218+, ext4, no problems.
Also, put 16GB of memory on those. Thay can take it and helps a lot.
In the mean time, search for posts about Fatal error. You need to modify some timeouts I believe. And see what logs tell you.
https://forum.storj.io/t/synology-memory-upgrade-guide/20743