I haven't seen an "official" explanation by mr. Johansson himself, but after reading some interviews with the other band-members I have figured out that he simply didn't have enough time for the band anymore. He wanted to spend some time with his family rather than rehearse and thus he had to leave the band. Sad but true, for he wrote some great music during his prime. I don't know if he's been involved with any other bands/projects.
And no, he wasn't the same Johansson who played on "December Flower" by In Flames and on Dimension Zero - that was just another guy with a same name.
-Villain (Dr. Tranquillity at your service)