[Satellite 6] subscription-manager returns 'Entitlement Certificate(s) update failed due to the following reasons: - unknown string format' error when trying to attach subscription for a custom product to a Satellite 6.x client

Issue

We created a custom product 'EPEL 8'. We try to attach the subscription for this product to a RHEL 8 system which is registered to Satellite 6.x server using the command:

# subscription-manager attach --pool 2c9abea46f13754501703932df576aa9

When we do so, the following message is returned:

Successfully attached a subscription for: epel 8
Entitlement Certificate(s) update failed due to the following reasons:
        - unknown string format

After attaching the subscription, yum update fails with :

Errors during downloading metadata for repository 'EPEL8':
  - Status code: 500 for https://satellite.example.com/pulp/repos/Org/Library/custom/EPEL8x/EPELxRepo/repodata/repomd.xml (IP: )
Error: Failed to download metadata for repo 'EPEL8x_EPEL8': Cannot download repomd.xml: Cannot download repodata/repomd.xml: All mirrors were tried