@Joost Schut - It sounds like someone may have changed the board’s Share settings since they sent the link. You will need to contact someone who does have access to the board and ask them to check the board settings. You will have to reach out to someone who does have access to the board to request access again.

There are a number of options of giving someone outside of the Team access to a board and depending on whether you need to edit the board or just view/comment on it, there may be costs.

Here’s Sharing Boards and Inviting Collaborators help article.