Hello Neil,my error was based on the network binding. I got the error if I made a mistake in the item configuration:
Switch NAME “NAME” { channel=“network:*device:*192_168_178_32:online” } WRONG
After new discovery it must be ( running config):
Switch NAME “NAME” { channel=“network:*pingdevice:*192_168_178_32:online” } OK!!!
AndI got the error too if the device was not reachable itself.
Looks like the PNG creating part was missing the data independent on the root cause. If the date was not there the error appeared.
I solved the topic first by changing to the right item configuration and by taking not reachable network devices out of the control.
Not sure what you did really. But it could be that the PNG file can not be created in addition to missing data too?
Regards