You will need to add 'debug_level = 9' within the [domain/mydomain.com] section of sssd, restart sssd and then provide the logs from /var/log/sssd/sssd_mydomain.com.log for the block of time when this happened:

Jul 10 17:37:47 MyIssueMachine sshd[42400]: pam_sss(sshd:auth): authentication success; logname= uid=0 euid=0 tty=ssh ruser= rhost=192.168.150.15 user=MyUser@mydomain.com
Jul 10 17:37:47 MyIssueMachine sshd[42400]: pam_sss(sshd:account): Access denied for user MyUser@mydomain.com: 4 (System error)
Jul 10 17:37:47 MyIssueMachine sshd[42400]: Failed password for MyUser@mydomain.com from 192.168.150.15 port 51594 ssh2
Jul 10 17:37:47 MyIssueMachine sshd[42400]: fatal: Access denied for user MyUser@mydomain.com by PAM account configuration [preauth]