The course "Seminário Integrador III" is listed at
https://pt.wikipedia.org/wiki/Special:Courses?uselang=en
but the "Enroll" link available at
https://pt.wikipedia.org/wiki/Ensino:Universidade_Federal_do_Rio_Grande_do_Sul/Seminário_Integrador_III_(2015/01)?uselang=en
causes the error
The course you tried to enroll for does not exist. See the list of courses.
This was originally reported at https://pt.wikipedia.org/w/index.php?diff=41709247 by @Lechatjaune.