Customer Record > Give Access > Contact Record > Manually Assign a Password > Unexpected Error: Lost Access

If the error is only received for a specific email address, the following are the steps in order to avoid receiving the error. 

1. Look for another contact record with Identical email address and contact name which is not related to any customer and it did not have any transactions, cases or other relations to any other records (This could be done via global search)
2. Purge/delete the duplicate record
3. Edit the Customer Record
4. Manually Assign a password
5. Save

You should be able to Save the Customer Record without receiving any error.