The fix that worked for me is found on this link
Resolution
There is an empty value on display style field for the announcement "ISC:Invalid Run As User".
Open the announcement "ISC:Invalid Run As User" which has a valid to date is either today or after.
The value for the display style filed is seems to be empty in the form but if you check in the xml it has a value of sys id "7f8fe758c31023002ff2554d81d3ae99" which does not exist.
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Change this value and add a value from the search; Then save it; Then delete the value again; Save it again; With that you are removing the invalid reference for "<display_style/>" from the xml. Basically you assign a valid value to Display style field, save it, then remove it, save it and the invalid reference is gone.
Do this for every "ISC:Invalid Run As User" from the list
Basically the steps to fix the error are these:
- In the service now (backend) - search for announcements - (Service Portal -> Announcements)
- Search "ISC:Invalid Run As User" in the column "Name"
- For every record with the name "ISC:Invalid Run As User" do the next step.

Note how there is a (i) button next to the empty "Display style" field - meaning there is some reference which in our case is invalid and should be deleted from the xml field - "display_style"
The value for the display style filed is seems to be empty in the form but if you check in the xml it has a value of sys id "7f8fe758c31023002ff2554d81d3ae99" which does not exist.
- Assign a valid value to "Display style" field (in my case 'Normal'), save it, then remove it, save it and the invalid reference is gone. Do this for every "ISC:Invalid Run As User" from the list in "Announcements"
Enjoy!
This fixed the error for me.