black screen on VT7 after /etc/init.d/xdm start


Reproducible: Always

Steps to Reproduce:
1. set DISPLAYMANAGER="sddm" in /etc/conf.d/xdm and CHECKVT=7
2. set /etc/X11/xorg.conf
3. /etc/init.d xdm start

Actual Results:  
Black screen in VT7

Expected Results:  
A login window

No error in Xorg.0.log
But a lot of errors in sddm.log below. (all sddm.log is in https://bpaste.net/show/70e48b19f609)

[23:31:52.303] (EE) HELPER: Received a wrong opcode instead of AUTHENTICATED: 0 
[23:31:52.303] (EE) HELPER: Unable to run user session: unknown session type 
[23:31:52.303] (EE) HELPER:  Auth: SafeDataStream: Could not write all stored data 
[23:31:52.303] (EE) HELPER: Received a wrong opcode instead of SESSION_STATUS: 0

[00:19:41.701] (EE) HELPER: Received a wrong opcode instead of AUTHENTICATED: 0 
[00:19:41.702] (EE) HELPER: Unable to run user session: unknown session type 
[00:19:41.702] (EE) HELPER:  Auth: SafeDataStream: Could not write all stored data 
[00:19:41.702] (EE) HELPER: Received a wrong opcode instead of SESSION_STATUS: 0

[23:35:44.592] (WW) DAEMON: QProcess: Destroyed while process ("/usr/libexec/sddm-helper") is still running. 
[23:35:44.594] (II) HELPER: [PAM] Starting... 
[23:35:44.595] (II) HELPER: [PAM] Authenticating... 
[23:35:44.595] (II) HELPER: [PAM] returning. 
[23:35:44.595] (EE) HELPER:  Auth: SafeDataStream: Could not write all stored data 
[23:35:44.595] (WW) HELPER: QIODevice::read (QTcpSocket): device not open 
[23:35:44.595] (EE) HELPER: Received a wrong opcode instead of AUTHENTICATED: 0 
[23:35:44.595] (EE) HELPER: Unable to run user session: unknown session type 
[23:35:44.595] (EE) HELPER:  Auth: SafeDataStream: Could not write all stored data 
[23:35:44.595] (WW) HELPER: QIODevice::read (QTcpSocket): device not open 
[23:35:44.595] (EE) HELPER: Received a wrong opcode instead of SESSION_STATUS: 0 
[23:35:44.595] (II) HELPER: [PAM] Ended.