Non clustered index on partitioned table
Your job most likely blew up because you added the non-clustered index to the base table, but did not add it to the staging table required for switching partitions in and out.
There are a significant number of things to consider when using partition switching - please read Transferring Data Efficiently by Using Partition Switching.  From that post.

Nonclustered indexes must be defined and must be identical. Any
nonclustered indexes that are defined on the target table are also
defined on the source table and are structured identically in terms of
uniqueness, subkeys, and the sorting direction (ASC or DESC) for each
index key column. Nonclustered indexes that are disabled are exempt
from this requirement.