Memory upgrade is what solved the problem, (+reinstall) enabling swap might have solved it too though.
[Solved] Changing Group Name Removes All Members
I just changed the name of a group on my forum and it removed all of the members from the group. I reverted the name change and all of the members are suddenly back in the group. It looks like the users are probably linked in the database by the group name, but changing the name on the edit group dialog does not update the linked user records.
This has been fixed on master but I don't think it is backported to 0.5.3 yet.
My ACP just told me 0.5.4 is out. This is obviously something I can work around, but I'll probably upgrade this weekend.
The change should have been backported to
0.5.x, so you can just pull the latest commits from that branch to get v0.5.4