Hi Brandon,
This error comes when a category assigned to a material and later someone deletes it, but in Material it remains the same. So when system tries to download the same material it throws the error that category does not exists.
The only solution for this find the material by giving this category entry change the category & start the replication again.
You need to abort the current initial load, delete the queues in crm (SMQ2/SMQ1) the initiate the load.
Regards,
Kamfrk,