I have a WordPress Site that is the SSO provider for Discourse using the WPDiscourse plugin.
I am not sure when it started happening but probably in the last couple of months, when a new user tries to register to the site they receive an activation link that looks like this without WP_Discourse plugin:
Are you sure this the only difference between seeing the error and not seeing the error? i.e. when you deactivate the plugin the error goes away immediately and when you reactivate it it comes back?
I tried downgrading DiscourseWP but the same happens, I wonder if there is something in recent releases of WP that might have impact in the mail_key verification by discourse.
@rodrigo.braga Thanks for reporting this. I was able to reproduce it. The fix is currently under review and will be in the next version of the plugin (2.5.1) which will be released very soon.