X11 forwarding request failed on channel 0

[Haanraadts, Mark (HBO) via cygwin]

I have a problem with remote ssh sessions to corporate Red Hat 6 servers and X11 forwarding. Xclock works perfectly fine on my local CYGWIn install. On a remote server I get:
$ xclock
Error: Can't open display:

Here's the debug shell logfile:

debug2: we sent a password packet, wait for reply
debug1: Authentication succeeded (password).
Authenticated to <host>.
debug1: channel 0: new [client-session]
debug2: channel 0: send open
debug1: Requesting mailto:no-more-sessions@openssh.com
debug1: Entering interactive session.
debug1: pledge: exec
debug2: channel_input_open_confirmation: channel 0: callback start
debug2: x11_get_proto: /usr/bin/xauth  list :0.0 2>/dev/null
debug1: Requesting X11 forwarding with authentication spoofing.
debug2: channel 0: request x11-req confirm 1
debug2: fd 3 setting TCP_NODELAY
debug2: client_session2_setup: id 0
debug2: channel 0: request pty-req confirm 1
debug2: channel 0: request shell confirm 1
debug2: channel_input_open_confirmation: channel 0: callback done
debug2: channel 0: open confirm rwindow 0 rmax 32768
debug2: channel_input_status_confirm: type 100 id 0
X11 forwarding request failed on channel 0
debug2: channel_input_status_confirm: type 99 id 0
debug2: PTY allocation request accepted on channel 0
debug2: channel 0: rcvd adjust 2097152
debug2: channel_input_status_confirm: type 99 id 0
debug2: shell request accepted on channel 0
Last login: Wed Sep 11 17:01:50 2019 from 10.218.16.112

Did a lot of Googling and I already changed this in /etc/ssh/sshd_config
X11Forwarding yes
AddressFamily inet
X11UseLocalhost no

Here's how I'm starting X-Windows
startxwin &
sleep 10
export DISPLAY=:0.0
xhost +
xterm -sb -sl 5000 -geometry 168x17+380+0 -T "XTERM 1" &
xterm -sb -sl 5000 -geometry 168x17+380+315 -T "XTERM 2" &
xterm -sb -sl 5000 -geometry 168x16+380+600 -T "XTERM 3" &



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