Hi Kevin
I will upgrade and report-back.
In response to your other questions:
Does this behavior appear when extensions are enabled or disabled?
I disabled all extensions, created a new template and copied the login form tag from EE docs, but when I tried to login I also get the “invalid username or password” message. The user/pass is a correct admin account so I know it works.
I tried with and without the ‘index.php’ in the url, and I tried different browsers. Same message.
I’ll upgrade and see what happens.
Thanks for your help.