Safari on Mac: "A problem repeatedly occurred" error

Safari 26.3 for Mac (Tahoe 26.3) returns message "A problem repeatedly occurred with <website>" and does not load the website.

For example this happens with "X" after I login, but does not happen if i access X using Safari on my iPhone. Yes, I've cleared cache, cookies, etc. ALL the previous recommendations. Does not happen to about half the websites. These include PayPal, and USPS (and many more)