It's possible that the way your server console program.. (I'm assuming HLSW) works it was mixing up the parameters for some reason. Here are a few steps to ensure that it works in the future.
1. Don't use the full name. ULX has partial name checking and you should use it to your advantage. 'Lemming' would have worked just fine.
2. Don't use quotation marks around the name and the group. they are unnecissary and however unlikely, it could cause the program to parse the info wrong.
If I had to wager a guess.. I'd say that SOMEHOW the name in it's length and use of non alpha-numeric characters was split and the later part of it was sent to the command as the group. Rather than receiving 'mod' as the group it was receiving the later part of the name. Just a theory... Try this in your console and see if it works.
ulx adduser lemming mod
I would bet that it would work perfectly. (assuming, of course, that there isnt another person on the server with lemming in their name at the time.)