Help with mysql error during sales invoice posting

Please can someone help with this? I have an installation in my shop, on submitting invoice it hangs on submitting and hitting f12 shows error 500 in the chrome console with a strange json error. At first I thought it was mysql packet setting but I observed I had raised max_allowed_packet to around 128M already, I also did set global max_allowed_packet to something very large. Can somneone look at the trace as shown in the attached and tell me what could be wrong?

Thanks in anticipation

Seems mysql went down! please check mysql error logs

Thanks @saurabh6790

I will check again, I have tried so many things, its such a small database so I wonder why this has happened, its barely 3 days up and just a few items.