Yea, it's real PITA and i got exact same issues when trying to vote for some package 1st time. Countless trying and total fail. Unfortunately i don't remember exactly what i did, but something reminds me that maemo.org package voting account may be unrelated to garage page voting (yea, very intuitive). Or at least logging process is different.

Try using Your e-mail as login. If that doesn't help, use Your logging with only lowercase characters. If that doesn't help, last resort is to create next account

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After initial problems, now, when i remember exactly voting user password and account I never got any problems with voting. so, don't get discouraged, fix problems and You'll be able to vote without any issues anytime.