I guess your model has a weak link to a function in UDF.xmod which it imports at execution?
Right, there's a lot going on here : firstly, DataPump, and normal ES usage, will call the model "live". They'll do it as their service account, and that account has to be able to read the model and any linked models.
C:/Users... should be readable on the server to any local admin and the service account should be a local admin.
ES can "pre-mine" data using a separate model - this is executed at data ingestion and not at data retrieval, so if this is not working you'll need to flush the data and re-ingest it.
Hope this helps,