Tomas,
Please check the language file for the SCLogin module in whatever language you're having issues with. For English, the string is:
MOD_SCLOGIN_WELCOME="Welcome %s,"
Please make sure the language file you're using has the same %s where the user's name should appear. If you still have problems, let us know what language is causing you issues and what the string is for the MOD_SCLOGIN_WELCOME key and we'll help however we can.
Thanks,
Alex