Hi,
I had same symptoms with UID:GID 0:0. Default ioFTPD account caused (i think) my crashes, same for users in GID:0.
Several times before the crash when i was doing a !who it was showing me most of my users in GID:0 as nobody group (in 4.1.5 and 4.5.4).
Another thing was when i did a !users, it was showing me multiple ioFTPD accounts (?!?) and then it crashed a few after that (this one was observed in 4.1.5 only).
My only try was to delete UID:0 account et GID:0 group. Now io runs for 36h without crashing wich is a record...let's cross my fingers