Got this reply from Discord:
Based on the information and what you've described, the most likely reason you can't join is that the server owner has paused invites. When invites are paused, anyone who tries to join will see a message and won't be able to access the server, even if the link appears valid and works for others who joined before the pause.
If you have a way to contact the server owner or moderators, I recommend asking them if invites are currently paused. Once they re-enable invites, you should be able to join using the same link.