Hello Ed,
Thanks for the suggestion, I tried with LightDM, which gave the same black screen, with the following in /var/log/messages, not sure why it's trying to talk to PulseAudio?
Aug 15 21:18:13 testlaptop dbus-daemon[1804]: [system] Activating via systemd: service name='net.reactivated.Fprint' unit='fprintd.service' requested by ':1.101' (uid=0 pid=3213 comm="sudo dnf install lightdm " label="unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023")
Aug 15 21:20:04 testlaptop audit[1]: SERVICE_START pid=1 uid=0 auid=4294967295 ses=4294967295 subj=system_u:system_r:init_t:s0 msg='unit=lightdm comm="systemd" exe="/usr/lib/systemd/systemd" hostname=? addr=? terminal=? res=success'
Aug 15 21:20:05 testlaptop audit[1560]: CRED_ACQ pid=1560 uid=0 auid=4294967295 ses=4294967295 subj=system_u:system_r:xdm_t:s0-s0:c0.c1023 msg='op=PAM:setcred grantors=pam_env,pam_permit acct="lightdm" exe="/usr/sbin/lightdm" hostname=? addr=? terminal=:0 res=success'
Aug 15 21:20:05 testlaptop systemd[1]: Created slice User Slice of lightdm.
Aug 15 21:20:05 testlaptop systemd-logind[1198]: New session c1 of user lightdm.
Aug 15 21:20:05 testlaptop systemd[1]: Started Session c1 of user lightdm.
Aug 15 21:20:06 testlaptop audit[1564]: USER_ACCT pid=1564 uid=0 auid=4294967295 ses=4294967295 subj=system_u:system_r:init_t:s0 msg='op=PAM:accounting grantors=pam_unix,pam_localuser acct="lightdm" exe="/usr/lib/systemd/systemd" hostname=? addr=? terminal=? res=success'
Aug 15 21:20:06 testlaptop audit[1564]: USER_START pid=1564 uid=0 auid=978 ses=1 subj=system_u:system_r:init_t:s0 msg='op=PAM:session_open grantors=pam_selinux,pam_selinux,pam_loginuid,pam_keyinit,pam_limits,pam_systemd,pam_unix acct="lightdm" exe="/usr/lib/systemd/systemd" hostname=? addr=? terminal=? res=success'
Aug 15 21:20:06 testlaptop audit[1560]: USER_START pid=1560 uid=0 auid=4294967295 ses=4294967295 subj=system_u:system_r:xdm_t:s0-s0:c0.c1023 msg='op=PAM:session_open grantors=pam_unix,pam_systemd acct="lightdm" exe="/usr/sbin/lightdm" hostname=? addr=? terminal=:0 res=success'
Aug 15 21:20:06 testlaptop dbus-daemon[1585]: [session uid=978 pid=1585] Activating via systemd: service name='org.a11y.Bus' unit='at-spi-dbus-bus.service' requested by ':1.1' (uid=978 pid=1573 comm="/usr/sbin/lightdm-gtk-greeter " label="system_u:system_r:xdm_t:s0-s0:c0.c1023")
Aug 15 21:20:06 testlaptop at-spi-bus-launcher[1592]: dbus-daemon[1600]: Activating service name='org.a11y.atspi.Registry' requested by ':1.0' (uid=978 pid=1573 comm="/usr/sbin/lightdm-gtk-greeter " label="system_u:system_r:xdm_t:s0-s0:c0.c1023")
Aug 15 21:20:06 testlaptop dbus-daemon[1187]: [system] Activating via systemd: service name='net.reactivated.Fprint' unit='fprintd.service' requested by ':1.50' (uid=0 pid=1643 comm="lightdm --session-child 12 19 " label="system_u:system_r:xdm_t:s0-s0:c0.c1023")
Aug 15 21:20:06 testlaptop pulseaudio[1572]: W: [pulseaudio] authkey.c: Failed to open cookie file '/var/lib/lightdm/.config/pulse/cookie': No such file or directory
Aug 15 21:20:06 testlaptop pulseaudio[1572]: W: [pulseaudio] authkey.c: Failed to load authentication key '/var/lib/lightdm/.config/pulse/cookie': No such file or directory
Aug 15 21:20:06 testlaptop pulseaudio[1572]: W: [pulseaudio] authkey.c: Failed to open cookie file '/var/lib/lightdm/.pulse-cookie': No such file or directory
Aug 15 21:20:06 testlaptop pulseaudio[1572]: W: [pulseaudio] authkey.c: Failed to load authentication key '/var/lib/lightdm/.pulse-cookie': No such file or directory
I also found this in messages with SDDM:
Aug 15 07:17:08 testlaptop sddm-greeter[10536]: QObject: Cannot create children for a parent that is in a different thread.#012(Parent is SDDM::GreeterApp(0x7ffd91c83a70), parent's thread is QThread(0x55fb84d792a0), current thread is QThread(0x55fb84e52a00)
Aug 15 07:17:08 testlaptop sddm-greeter[10536]: QObject: Cannot create children for a parent that is in a different thread.#012(Parent is SDDM::GreeterApp(0x7ffd91c83a70), parent's thread is QThread(0x55fb84d792a0), current thread is QThread(0x55fb84e52a00)
Aug 15 07:17:08 testlaptop sddm-greeter[10536]: QObject: Cannot create children for a parent that is in a different thread.#012(Parent is SDDM::GreeterApp(0x7ffd91c83a70), parent's thread is QThread(0x55fb84d792a0), current thread is QThread(0x55fb84e52a00)
Aug 15 07:17:08 testlaptop sddm-greeter[10536]: QObject: Cannot create children for a parent that is in a different thread.#012(Parent is SDDM::GreeterApp(0x7ffd91c83a70), parent's thread is QThread(0x55fb84d792a0), current thread is QThread(0x55fb84e52a00)
Aug 15 07:17:08 testlaptop sddm-greeter[10536]: QObject::installEventFilter(): Cannot filter events for objects in a different thread.
Aug 15 07:17:08 testlaptop sddm-greeter[10536]: QObject: Cannot create children for a parent that is in a different thread.#012(Parent is SDDM::GreeterApp(0x7ffd91c83a70), parent's thread is QThread(0x55fb84d792a0), current thread is QThread(0x55fb84e52a00)
Aug 15 07:17:09 testlaptop sddm-greeter[10536]: QObject: Cannot create children for a parent that is in a different thread.#012(Parent is SDDM::GreeterApp(0x7ffd91c83a70), parent's thread is QThread(0x55fb84d792a0), current thread is QThread(0x55fb84e52a00)
Aug 15 07:17:09 testlaptop sddm-greeter[10536]: QObject::installEventFilter(): Cannot filter events for objects in a different thread.
Aug 15 07:17:09 testlaptop dbus-daemon[10561]: [session uid=995 pid=10561] Activating service name='org.gnome.GConf' requested by ':1.3' (uid=995 pid=10587 comm="/usr/libexec/pulse/gconf-helper " label="unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023")
Aug 15 07:17:09 testlaptop dbus-daemon[10561]: [session uid=995 pid=10561] Successfully activated service 'org.gnome.GConf'
Aug 15 07:17:09 testlaptop sddm-greeter[10536]: Cannot watch QRC-like path ":/icons/hicolor/index.theme"
Googling suggested a few different things to try, nothing seemed to work in this case, but for reference:
sudo rm -fr /var/lib/sddm/.cache/sddm-greeter/qmlcache
Tried both Breeze themes, then installed sddm-theme and tried one of those.
Tried with a monitor plugged into the HDMI port
There does appear to be a block of text flashing up just before the screen goes blank, it's an ACPI error or two I think, but it flashes by and I can't find it in the logs.
I'll see if I can grab a photo of it so I can see what it says,
Thomas