Cannot Login or Reset Password

I first installed this plugin on a client site http://www.karenkaybuckleyaustralia.com.au a year ago and it has been working well until the end of March. (I updated to version 1.3.83 in early March without incident.)

Now UM Users cannot login or reset their password via the UM default login page. After you input your username and password, you are simply presented with the login page.

Similarly, for users trying to reset their password, they receive the reset password link but after they input their new password they are presented with the password reset page.

In terms of troubleshooting, I have tried the following steps:
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• I have changed to the Twenty Fifteen theme and deactivated all plugins except UM.
• using earlier versions of UM saved from back ups (1.3.72 and 1.3.82).
• changing settings in UM>Forms>Default Login>Edit>Options>Redirect After Login

In each case, the login/reset problem remains.

I welcome suggestions of other steps I should try to resolve this issue.