The url generated by discourse contains upper and lower case. That url is case sensitive. (i.e. if the case is changed the correct page does not load). Some browsers will change everying to lower case thus breaking the link.
If upper case is not necessary I suggest generated links be all lower case to avoid specific browser issues or make sure that the server handles mixed case as lower case.
FYI, I see the link in an activation email has no upper case so not an issue there just with invites.