Have a look at Debug and fixing common DiscourseConnect issues. If you can not sort out the problem yourself, post a redacted version of an error log entry here, someone may be able to help.
I do not think the error you are reporting is a Discourse bug.
If you disable DiscourseConnect, users who have created accounts on Discourse via your WordPress site will have to click the “I forgot my password” link from the Discourse login modal and use the email they are sent to reset their password.