Did this user do anything before this issue occurred?
Based on my knowledge, after creating Exchange, three self-signed certificates will be automatically generated, among which Microsoft Exchange self-signed certificate to encrypt network traffic between Exchange servers and services.
For more information:Certificates in Exchange
Once we enable a service for the certificate, we cannot disable it. We could only re-import a new certificate, assign the started service, and then delete the old certificate. Considering that deleting a self-signed certificate may cause other effects, it is recommended that you run the following command line to export the certificate after confirming that the service has been enabled on the new certificate. Then please run the IISreset in CMD started as administrator and see if the issue is solved.
Export-ExchangeCertificate -Thumbprint <> -Server <> -FileName "<>"
For more information:export-exchangecertificate