The Mid Server will lose data if essential fields on the ecc_queue table have been made inactive or removed.
You could be experiencing this problem as well if the same mid-server works fine when connected to another instance.
Steps to Reproduce
1 - Open the ECC queue table and disable the payload field (set active = false).
2 - Save.
3 - Create a REST message that uses username/password.
4 - Validate the message works without the mid-server.
5 - Modify the REST message to use the mid-server.
The mid-server returns a "User Not Authenticated" error for missing username/password, instead of validating the ecc_queue containing all the active essential fields, to prevent data loss.
This issue is targeted to be fixed in the London release.
As a workaround, please validate the ecc_queue table and review it contains the essential fields with active set to "true":
Related Problem: PRB1105662