Username already exists

Thanks for your time; this is new within a week and I have nothing new of different on site: new user gets sent a unique url to create membership and at the end gets a “There has been a critical error on this website.”

I checked the WPM log:

[2021/03/27 09:29:50] - SUCCESS: WooCommerce Simple Membership integration - Order processed... checking if member account needs to be created or updated.
[2021/03/27 09:29:50] - SUCCESS: WooCommerce Simple Membership integration - Order status: pending
[2021/03/27 09:29:50] - SUCCESS: WooCommerce Simple Membership integration - Order status for this transaction is not in a 'completed' or 'processing' state. Membership update won't be handled at this stage.
[2021/03/27 09:29:50] - SUCCESS: The membership account creation or update for this transaciton will be handled when you set the order status to completed.
[2021/03/27 09:29:53] - SUCCESS: WooCommerce Simple Membership integration - Order processed... checking if member account needs to be created or updated.
[2021/03/27 09:29:53] - SUCCESS: WooCommerce Simple Membership integration - Order status: processing
[2021/03/27 09:29:53] - SUCCESS: First name: XXX, Last Name: CCCCC, Email: [email protected]
[2021/03/27 09:29:53] - SUCCESS: Membership Level ID (3) is present in this product. Processing membership account related tasks...
[2021/03/27 09:29:53] - SUCCESS: Invoking Simple Membership user signup handling function.
[2021/03/27 09:29:53] - SUCCESS: swpm_handle_subsc_signup_stand_alone(). Custom value: , Unique reference: 27668
[2021/03/27 09:29:53] - SUCCESS: Could not find any record using the given email address ([email protected]). Attempting to query database using the unique reference: 27668
[2021/03/27 09:29:53] - SUCCESS: Did not find a match for an existing member profile for the given reference. This must be a new payment from a new member.
[2021/03/27 09:29:53] - SUCCESS: Creating new member account. Membership level ID: 3, Subscriber ID value: 27668
[2021/03/27 09:29:53] - SUCCESS: Member signup URL: https://www.DDDDDDDD.com/membership-join/membership-registration/?member_id=685&code=cd5d441626b28fc9f0c005025952479a
[2021/03/27 09:29:53] - SUCCESS: Member signup (prompt to complete registration) email successfully sent to: [email protected]
[2021/03/27 09:32:08] - SUCCESS: Simple Membership user account with this username already exists! No new account will be created for this user.

This is a new member so not sure why it says it already exists.

Has happened to all new users.

Thanks so much!