Sorry about delay....
"Tarefa" sounds almost good for me... Marcello is right when say "widely used in `Project Management`"... but, Project management, is in fact, just one of the purposes of Redmine, it is essentialy a Bug/Issue Tracker or a Ticket Managment System.
I found some (little) discussion about it here: http://por.proz.com/kudoz/english_to_portuguese/computers:_software/1372809-issue.html
other snippet I found here:
" Tradução da palavra issue para o português:
s. e v. emissao, edicao, questao, saida, progenie, publicacao, acontecimento; emitir, publicar, editar, expedir, proceder, emanar"
in the last snippet the word "acontecimento" seems to be the most compatible with the scope of Redmine, but I think is not good too.
going along I found:
"s.m. Evento, o que acontece: acontecimento feliz. Sucesso, eventualidade, fato."
we can see some relation between issue and "evento" or "fato".
Trac people are using "Tíquete", in this case I prefer 'Ticket' in English... by the same reason the most Brazilian keyboards haven't theirs keys translated (Enter, Backspace, Shift, Caps Lock, Home, End, Pg Dn...)