Issues pulling lookup fields for RETS Update

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Hello, we are attempting to pull the lookup fields for Highland Lakes from the Partial Listing Resource for RETS Update purposes. We need to pull the lookups from all 5 classes but are running into problems. Everytime we try to pull the lookup fields one or more of the classes are failing with a 20502 error telling us the update template is invalid. It is never the same class that causes the error. Sometimes 4 of the 5 will work. Sometimes only 1 will work. Thanks.