Thanks for suggestions.
It only happens for this WordPress site, as I regularly login to plenty of other sites without problem.
I’m not using a mobile app, just Safari or Chrome on my Mac. I’ve tried typing the password manually, copy and pasting it, and using the password stored KeyChain. I double check that it is the correct password I.e. it hasn’t reverted to an old password in the login box.
So once I reset the password via email, it’s usually fine for most of the day. During that time, I can even log out and back in again without problem.
But then I come back to login the next day and I’m told I’ve entered the wrong details. Reset. Go again.