Octoprint Server: Loading Failed

What is the problem?
After printing for a couple of hours and monitoring the process on my browser in a separate tap, I noticed while trying to check the process that the tap only showed "Loading Failed". Since the
print was almost done, I waited for it to finish and just rebooted the raspberry pi3 via ssh.
I started another print and half an hour into the print the same thing happened.

I printed all day yesterday and had no problems at all.

What did you already try to solve it?
see above. I also restarted the browser and tried other browser and other computers to connect from.
I always get the login prompt and can type in my credentials, after that I get the Connecting to server
page with the spinning wheel and then "Loading Error". I cannot reboot now, because the print will be 4 more hours.
Logs (octoprint.log, serial.log or output on terminal tab at a minimum, browser error console if UI issue ... no logs, no support!)
No serial log turned on. octoprint,log from the second print:

Additional information about your setup (OctoPrint version, OctoPi version, printer, firmware, browser, operating system, ... as much data as possible)

OctoPrint version : 1.4.0
OctoPi version : 0.15.1

Here is the log now:octoprint.log (117.3 KB)