Sending, receiving, and organizing email in Outlook.com
I have the same problem. It occurs in both Firebox and Chrome browsers. Clearing the cache and saved form data does not help. Logging in and out of my Windows account did not help. None of the suggested measures I have seen helps.
I was able to use the Outlook Classic application in Windows 11 to edit the Safe Senders and Blocked Senders lists. Expand the ribbon and look under the Delete section to find the icons to edit the lists.
I removed quite a few entries in order to check if the lists had reached maximum size. I could see these changes in the browser version of Outlook. However, reducing the sizes of the lists DID NOT fix the issue with saving changes in the browser version. Using the desktop version does at least let you fix mistakes if you accidentally added an entry to a list which you need to remove.