Gmail "Discard changes" popup

Gmail (for my postjobfree.com webdomain) surprised me again today.

I went to "Forwarding and POP/IMAP" setting
https://mail.google.com/mail/u/0/#settings/fwdandpop

I selected "Enable POP for all mail" radiobutton.
Then I clicked "Save Changes".

Gmail showed "Saving" label for about 15 seconds.
Then Gmail changed "Saving" label to "Still working ..." label.
Then after another 10 seconds of deliberation, Gmail showed me "discard" popup (I was patiently waiting and did not discard anything)
~~~~~
Confirm discard changes

Your changes have not been saved. Discard changes?

[Cancel] [OK]
~~~~~

"Cancel" click keeps Gmail "Forwarding and POP/IMAP" setting screen in "Loading..." state.
It hangs in that state for, at least, 20 minutes.

Similar issue is described here:
https://support.google.com/mail/thread/42859873?hl=en

I already enabled "Less secure app access" in that Gmail account.
That did not help.