Yesterday we were made aware of a bug in the Koofr share component by our users. After investigating we have indeed confirmed, that since the end of February, emails sent when user would share something with a non-user inside Koofr (through the Add people function) contained an invalid activation token. This prevented the invitee from creating an account directly from the invite link, but would have to go through the signup page again.
The problem was fixed, and all newly issued invite links are now again working properly. If you notice some users not accepting an invite in the last two weeks, you should re-invite them (you can simply remove and add them back to the same shared folder or file).
We apologise for the problems this may have caused and thank the users who notified us about the issue. Please note that regular signup and inviting friends through our Get more free space initiative was not affected, as were not the public links for sharing (Send link and Receive files options).