I have had several reports from people using Strato Hosting ( https://www.strato.de/ ), specifically in the area of the Netherlands, stating that not every user is added to the DOIFD subscribers table.
What happens is, the user goes through the sign-up process, they receive the verification email, but when they click the link, they receive an error message from DOIFD, stating "Bad Verification Number". This is happening because the users information is not added to the database.
The problem seems to be intermittent and may have to do with the location of the user (Firewall?). The servers generate no errors so it appears to be a intermittent database connection issue within the servers themselves.