After upgrading our Moodle from 1.9 to 3.3, most users experienced problem with unwanted popup window with text "Undefined" in it. I also saw that on my machine few times. Recently we upgraded to the latest Moodle 3.3.4+ (Build: 20180223). Now I can't reproduce this anymore but some users still have the same problem form time to time. It doesn't happen all the time so it's hard to catch and trace. The theme is default - Boost.
I've read this https://tracker.moodle.org/browse/MDL-58310 related to the similar or the same issue. It says it is solved from Moodle version 3.3.1. Some people mentioned custom user profile fields are somehow related to this. We have few also.
Does anyone else have this problem? Where to start looking for the cause? Any ideas?
Zoran