Wrong Invitation Code

First – thanks for the plugin! It works fine, mostly, but some codes that are created later don’t work – I got “Wrong Invitation Code’ for these. If it matters, I currently have ~ 1000 invitation codes. The code shows up both in “Invitation Codes List” and the table of Invitation Codes.

Any idea why this is happening?
https://www.ads-software.com/extend/plugins/baw-invitation-codes/