OJS font-end submission error

When someone makes a submission to our journal, the buffering wheel never stops spinning (never, even after a few hours). This is despite the submission having been made and registered as a submission, with automatic emails having been sent.

This is the error message:

Uncaught TypeError: right-hand side of ‘in’ should be an object, got undefined
ajaxErrorCallback https://journal.networkonculture.ca/js/build.js?v=3.4.0.5:344
error https://journal.networkonculture.ca/js/build.js?v=3.4.0.5:344
jQuery 4
build.js:344:102914
ajaxErrorCallback https://journal.networkonculture.ca/js/build.js?v=3.4.0.5:344
error https://journal.networkonculture.ca/js/build.js?v=3.4.0.5:344
jQuery 4

We’re using OJS 3.4.0.x

Any suggestions?