Investigating this, i found out that that particular user is an existing user, and is already registered with the relevant Patreon email. So when logging into the site via Patreon without first logging in to WP site as this user, it causes an exception since the plugin cant identify that this user is Patreon linked (it isnt), and then tries to create a user with a new username but with the same email. Which seems to cause an exception in user creation logic.
In any case this is relevant to the issue in the below thread, and a fix will be in the works by tomorrow.