This happened to me and the user B3N. It seems that if the word "Pokemon" is in the link, uncapitalized, it gets automatically capitalized and the link gets ruined.
http://www.serebii.net/black2white2/nspokemon.shtml
That link up here is meant to have a lowercase "p" in "Pokemon". When I post it, the link somehow turns the p -> P and the link is wrong.
This problem seems to be only happening on Pokebase, since it works here and Battle Subway.
♠SpumWack♠ and starpower saw the problem.