It seems to be pretty consistent. It can be reproduced after a few calls
The fact that the problem is "pretty consistent" is a game changer. Honestly: If it was a once every now and then thing, it would be very difficult to track down. It means you should be able to reproduce the problem using only TEST calls!
Now try it without any of your changes. Also try it with a different browser.
You can also have a look at the agent's javascript console to check for errors before going into full "debug trace" mode, which is time-consuming but always actually works.