I did not write RosterLib-2.0. And since its been used since 2006 without errors by a large number of other addons, i would guess that this is a private server related bug. The server might be sending conflicting information to the client causing this error. I would suggest using something like ImprovedErrorFrame. Also i would guess this error doesn't happen very often, does it?

Yep, only once on Nost PvE, right after I joined WSG. Maybe somehow server messed sending bg players.