I've got a Dell XPS 15 9570 with a TB16 docking station. Ubuntu works fine with 3 monitors (1 built-in + two external). However, strange things appear when I add a 4th monitor.
The Ubuntu sysconfig recognizes the monitor. In Devices -> Displays I can see all 4 monitors listed in the drop-down. However, I can only activate (put the graphical switch on "ON") of maximum 3 of them. Trying to activate the 4th, I get:
gnome-control-c[18797]: Config not applicable: GDBus.Error:org.freedesktop.DBus.Error.InvalidArgs:
No available CRTC for monitor 'SHP 0x148d' not found
I've researched this error, but didn't find anything reasonable. However, it has to be some hard-coded check somewhere?
I've seen there seem to be an 8192 pixel maximum virtual desktop size somewhere, but "glxinfo -l | grep GL_MAX_TEXTURE_SIZE" returns 16384 in my system.