This issue happens when the http activities were used in the workflows and mainly specific to workflow manager component in SharePoint 2013. more details as below:
Workflow Manager Certificates
Services SSL Certificate.
This is used for communication between Workflow Manager and its clients (for example SharePoint). When we register a Workflow Service with a SharePoint farm or otherwise interact with Workflow Manager, this is the bad boy that counts.
- This is a Server Communications Certificate.
- By default, its common name will be the FQDN of the first server in
the farm (e.g. CN=FABWFM1.fabrikam.com).
- This certificate includes a Subject Alternative Name for the DNS
domain (e.g. DNS Name=*.fabrikam.com).
- By default, this certificate is issued by the same FQDN. This is
effectively a Self-signed Certificate.
- When additional servers are joined to the farm, this certificate is
copied to the Trusted Root Certificate Authorities store.
Encryption Certificate.
This is used for encryption of connection strings stored in the Workflow Manager Management Database.
- This is a Server Communications Certificate.
- By default, its common name will be the FQDN of the first server in
the farm. This certificate includes a Subject Alternative Name.
- By default, this certificate is issued by the same FQDN.
- When additional servers are joined to the farm, this certificate is
copied to the Trusted Root Certificate Authorities store.
Outbound Signing Certificate.
This is used for securing communications between workflows and their clients and between different workflows (not between Workflow Manager and its clients as documented). This certificate is used to sign the security token portion of a HTTP activity, which includes the claims of the user that instantiates the workflow.
- This is a Certificate Signing Certificate.
- By default, its Common Name will be CN=WorkflowOutbound.
- By default this certificate is a CA Root Certificate so it’s issued
by itself.
- This certificate is created when the first host is added to the
Workflow Manager farm.
Reference Link