I linked you that to tell you to remove the special characters from members usernames and it will fix the problem and in admincp/settings/options/User Registration Options/Username Regular Expression put ^[a-zA-Z0-9\s.\-_']+$
Or something like this to stop members registering with things like this if you already have this filled, maybe what you have in there is wrong
On another note, maybe the dev has a way around to allow them if it has anything to do with this mod, but I think it is more mysql does not allow them
regexp is short for Regular expression
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