Public key rejected by remote server that is running DMVMOVE5.3.3

We are using WINSEP (5.17.8 (build 10803)) on a Server 2016 server to connect to a remote server that is running DMVMOVE5.3.3. When we are logged in as the "Administrator" to the server and use WINSEP to connect to the SFTP site with no problem. When one of our developers loginto the server and use WINSEP to connect to the SFTP site they get:
Searching for host
Connecting to host
Authenticating
Using USERNAME (the same that the administrator connection is using)
then they get
Server refused our key
access denied

The PPK Key was supplied to us by the remote DMVMOVE5.3.3 server administrator and imported into WinSEP

Please advise
Thanks in advanced

I assume you mean "WinSCP", not "WINSEP".
Please post WinSCP session log files from both scenarios.