Mods with "missing Item GameObject" (or similar) errors unfortunately require an update from the mod creator. Until then, certain custom items and objects may be missing from levels and servers.
In the cases we've reviewed, the issue was event-related components referencing a missing target object. For example, an event might be configured to trigger a particle system, but if the particle system was later removed without updating the event, the event is left pointing to an object that no longer exists.
Unfortunately, code-wise this is indistinguishable from an attempt to exploit the vulnerability, so it must be blocked.
For modders: we recommend checking the error log file. Above the missing GameObject warning should be a message with the precise component and event causing the issue.