The same fix that was applied to PvE is now active in Zombies.
1.4.1 added several new buff variants including but is not limited to: On Fire(Magma Stone), Frostburn (Frost Armor). However, the server currently uses 18.104.22.168 TShock for compatibility, which means it is behind in terms of content. This lead to the TShock buff dictionaries being outdated, and in turn kicking players with the infamous “Abnormally added buff to NPC”.
A fix has been applied to 1) update buff dictionaries, and 2) block non-existing buffs. This means that when you try to apply On Fire with Magma Stone, it will be blocked and the buff will be removed.