The following support KB article helped for me: https://support.servicenow.com/kb?id=kb_article_view&sysparm_article=KB0961053

if you are unable to access it, it says that there is a missing record in the sys_activity_supplemental table. the record needs to have an empty (NULL) document ID, Event ID, and User ID which applies globally. The record should contain a JSON as shown below:
{"background": {"subtype": {"work_notes": "yellow"}}}

Hope this helps for you!