After importing data from vBulletin 4 the members who used umlauts (äöü) and/or special characters (ß) in their passwords can't log in anymore. The strange thing: The 'password" table collation is "latin1_swedish_ci" in vBulletin 4, although the language of the forum is German not Swedish. (I think the problem happens while converting from "latin1_swedish_ci" to "utf8_general_ci".)
This is sort of tricky because technically speaking, the characters in the password aren't actually the special characters, due to the client-side hashing that vB does. They're likely not actually characters that you can type.
Unfortunately, it's not something that we can workaround. The only option is to reset the password for those users.