For what it’s worth I’m having the same problem. I installed Foreman v1.24 last week, with Centos 7.7 Clients, and get the exact same problem of clients not seeing any subscribed products, & error in the /var/log/rhsm.log:
File “/usr/lib64/python2.7/site-packages/rhsm/certificate2.py”, line 113, in _read_x509
raise CertificateException(str(e))
CertificateException: unknown string format