Many thanks, Ivar.

Developer Tools > Network showed status: 200 (from cache).

I then did <three dots> > Settings > Privacy and security > Delete browsing data. Clicked "Cached images and files". Unclicked the others. And then clicked "Delete data".

It's good now. Thanks.

(Though I wonder how ltd.foo.com got into this state but sub1.foo.com did not. Not a big concern at the moment.)