I have the same problem.
The export makes it through Recompressing images, Preparing project data and creating zip but sometime during the Build Service: Connected to server the message changes to
Build Service: Lost connection to build server.
Then the Log says Websocket is not closed
For trial and error purposes I tried on Chrome and Firefox on PC and Chrome on Mac.
The odd thing is randomly it will work but lately, more than not, I get the error. Happens in r323 and r324
Does anyone have a solution?