Thank you for this answer. I’ve tested your plugin on two installations:
1) PHP 5.5.1 / MySQL 4.1.13
2) PHP 4.4.2 / MySQL 4.0.25
On both installation there is the same problem. Before upgrading to WordPress 2.0 (and Smartlink 2.2) there was no problem on both installation.
Now I have tested german “Umlaute”, no errors with this:
[test ü??->this-is-a-slug]
Programming with PHP and MySQL is not my main profession, but I don’t think this problem has something to do with charsets, because uppercase is also ASCII and german “Umlaute” works fine here.
Are there any tests I can do for you on my servers helping to find this bug?