When importing constituent update batches an error is sometimes given regarding a lookup ID being invalid. This only happens for some records and may be related to customizations in the batch using out-of-box code elements.
We're currently evaluating this issue for a fix in a future patch or service pack.
Steps to Duplicate
1. Open the Batch 2. Select the Filters button 3. Filter on the error message "UC_Constituent_Lookup" 4. Clear all of the messages in this batch and validate - The error reappears on validation 5. Although this batch has custom elements this error is called from out of box code.