I did a conversion from IPB 3.4 to XenForo and found that a percentage of members were unable to authenticate on their first login attempt. But, many users could authenticate fine. Took a while to figure out, but, the information on how passwords are treated in IPB is explained here: https://www.invisionpower.com/support/guides/_/advanced-and-developers/integration/login-modules-r42 In the middle of that page is this: IPB actually runs the plain text password through that cleaning routine before doing the hash operation. I confirmed it by manually generating the hashes with and without cleaning the password first. The XenForo module that authenticates IPB login credentials does not take this into account. It just does the basic: return md5(md5($salt) . md5($password)); operation. I made an edit in: ./library/XenForo/Authentication/IPBoard.php - adding a function to do the same character substitutions prior to testing the hash, and that addressed the problem. When researching this, I was surprised it wasn't found before this. There are a couple threads here about passwords not always working on some IPB imports, but, nothing about this.