Problem with screencapture of flash animation in chromium-browser

I'm using chromium-browser and a vnc4 server to do some thumbnailing of flash resources. This is the version:

$ chromium-browser --version
Using PPAPI flash.
Chromium 48.0.2564.116 Ubuntu 14.04

This is what I do when I try to do the capture:

sudo vnc4server  -kill :1
vnc4server 
export DISPLAY=:1
sudo xhost +local: 
chromium-browser "<url to load the resource on our website>"

This has worked in the past; I would proceed to do a couple of interactions (pressing play buttons etc), then do a screengrab. But now it's hanging, displaying this output:

Using PPAPI flash.
Xlib:  extension "XInputExtension" missing on display ":1".
Xlib:  extension "XInputExtension" missing on display ":1".
Xlib:  extension "XInputExtension" missing on display ":1".
[3121:3121:0322/164259:ERROR:logging.h(808)] Failed to call method: org.freedesktop.DBus.ObjectManager.GetManagedObjects: object_path= /: org.freedesktop.DBus.Error.ServiceUnknown: The name org.bluez was not provided by any .service files
[3121:3121:0322/164259:ERROR:logging.h(808)] Failed to call method: org.freedesktop.DBus.ObjectManager.GetManagedObjects: object_path= /: org.freedesktop.DBus.Error.ServiceUnknown: The name org.bluez was not provided by any .service files
Xlib:  extension "XInputExtension" missing on display ":1".
libGL error: failed to load driver: swrast
[3176:3176:0322/164300:ERROR:gl_surface_glx.cc(347)] GLX 1.3 or later is required.
[3176:3176:0322/164300:ERROR:gl_surface_x11.cc(56)] GLSurfaceGLX::InitializeOneOff failed.
[3176:3176:0322/164300:ERROR:gpu_child_thread.cc(257)] Exiting GPU process due to errors during initialization
[3121:3155:0322/164300:ERROR:browser_gpu_channel_host_factory.cc(132)] Failed to launch GPU process.
[WARNING:flash/platform/pepper/pep_module.cpp(63)] SANDBOXED
Vector smash protection is enabled.

Now, apart from hanging the terminal, this does seem to be working. I can open another terminal and do the interactions and screengrabs in there just fine, like so:

#click play button
sudo xdotool mousemove 160 655; sudo xdotool click 1
#screengrab
sudo import -display :1 -window root -crop 800x600+11+83 /path/to/output.jpg

So, maybe I can actually ignore this problem if I find a way to bypass the hanging command. I tried adding an & on the end, like so:

chromium-browser "<url to load the resource on our website>" &

But it's still hanging. It's also hanging if I do

chromium-browser "<url to load the resource on our website>" >/dev/null

I've also tried starting it with nohup in front of the command (still hangs, but with no output) and nohup at the start with & at the end (same).

I'm out of ideas. I'm not familiar enough with vnc4 or chromium-browser to know what this error even means, let alone try to fix it. Can anyone advise?