The "Insert web link" button on the HTML field is stuck if the HTML editor is set to htmlArea. There is an exception in the browser console logs:
Uncaught TypeError: Cannot read property 'documentElement' of null
Notice “Insert/Modify Link” pop-up keeps loading
After carefully considering the severity and frequency of the issue, and risk of attempting a fix, it has been decided to not address this issue in any current or future releases. We do not make this decision lightly, and we apologize for any inconvenience. If you have any questions regarding this problem, contact ServiceNow Customer Support.