Hi Marcus,
If your DTM account is enabled for the Marketing Cloud, but the login company for the new invite is a legacy company, the account might be the issue.
First, try accepting the link by logging into the DTM account through DTM (not through the Marketing Cloud). If you receive an error that the password is wrong, use the password reset option.
If the account was created through the Marketing Cloud and was not ever used to log in directly at dtm.adobe.com, a new email address must be used. This is only the case if the account was created via marketing.adobe.com and has never logged in via dtm.adobe.com. This scenario occurs when the invite comes from a legacy company that has not migrated to the Marketing Cloud and the user was created through the Marketing Cloud.
For more information, Please see the note in Manage Users
Thanks & Regards
Parit Mittal