All possible icons are displayed on the JEditor's toolbar; changing toolbar settings in JEditor Configuration > Toolbar doesn't affect anything.
JEDITOR_CONFIGURATION_JIRA is undefined error appears in the browser console.
JEditor conflicts with a module of another add-on - Insight. This module is also an editor that loads its own configuration and breaks JEditor.
Disable the module that causes the conflict
- Go to JIRA administration > Add-ons > Manage add-ons
- Locate Insight and expand add-on's list of modules
- Find module named CK Editor(ckeditor) and disable it
After that JEditor should start working properly. There will be a minor impact on Insight's textarea - it may be displayed as a plain text field or as a JEditor's instance.