Inconsistent behavior when clicking More Applications due to some JS errors on Tomcat

STEPS TO REPRODUCE

1. Login as Admin
2. Click on "+ More applications", below the Applications panel
3. Create a subwiki
4. On that subwiki, click on "+ More applications", below the Applications panel as well

EXPECTED RESULTS

The menu extends, revealing two sub-menus: "Create your own!", respectively "Install new applications".

ACTUAL RESULTS

Sometimes, when clicking on "+ More applications", the user is directed straight to "Global Administration: Add Extensions" page and in browser's console there is the following error:

 Uncaught SyntaxError: Unexpected token '<'

Other times, on subwiki, when clicking "+ More applications", nothing happens, and in browser's console there is the following error:

Uncaught ReferenceError: Effect is not defined
    at klass.toggle (<anonymous>:1:723)
    at klass.<anonymous> (<anonymous>:1:467)
    at HTMLAnchorElement.<anonymous> (prototype.js?r=1:417:23)
    at HTMLAnchorElement.<anonymous> (prototype.js?r=1:7266:15)

These errors could not be reproduced consistently though and they seem to reproduce only on Tomcat from my tests.

Also, it seems that they reproduce also on older XWiki versions (tested on XWiki 13.10.3 and 12.6.8), but they don't seem to reproduce on XWiki 14.2.1.