Cannot login to admin

Tried to login to my remote admin panel as I hadn’t checked for update for 3 weeks and I get login fail.
Thinking I must have changed the password without updating password manager I renamed admin.yaml and got the missing admin user warning and form.
When I enter details, on create user the form clears and I get:
Just in case it was a windows/local network issue I repeated with my iPad and via a VPN connection to rule out local network security, same failure.
Finally I copied admin.yaml to my local development site copy and all aspects worked.

My site works perfectly, seems that plugins are ok including the contact form.
Not sure where to go from here as I do not know of anything that has changed since my last login 3 weeks ago. Anybody any ideas what I can do next.

Some times server updates messes permissions, so they have to be fixed, so
check that, as well as for the tmp directory of your hosting account root , where session is stored. Use apache for group.