When users try to enroll, they are getting A901 error
Check that the RAServicePool users is not locked or that the password has not expired.
One possible cause is that the password for the User running the RA Service has expired or has been reset.
Update the password, and if security policies allow, you can set the password to never expire to avoid this issue in the future.
Otherwise, review your PKI Enterprise Gateway logs and contact DigiCert Technical Support for assistance.