Double check that you don't have an extra space at the beginning or end of the invitation code in link in the emails that are being sent (if so you'll need to edit the Workflow that is sending the invitations), also if you have multiple environments double check the URL is to the correct Portal/Pages site.
You can also try creating a new Invitation, Save it, then copy and paste the code it generates into the Redeem Invitation field on the Portal to see if it is accepting it directly.
