Contrary to similar rumors that have been circulating for some time without any reliable source to support them, this information is trustworthy, as Mision Tokyo was the first website to confirm the release of Season 1 back in 2012, and the recent information was given to them by Selecta Visión, the official distributor of the anime in Spain (and the company that originally revealed the episode list of Junior High the last year), which in turn received it directly from Kodansha.
In their original blog post they have included part of the audio interview recorded during the 34 Salon del Comic in Barcelona where Selecta Visión marketing director Manu Guerrero reveals the delay, and the Facebook page of the company confirms that the interview indeed took place.
As stated by the source, Selecta Visión contacted Kodansha requesting the rights to simulcast Attack on Titan Season 2 in Spain during this Fall lineup, but they were told by the Japanese company that it will not be airing this year and that it has been rescheduled to some point of 2017, so the distributor estimates that it will premiere during the anime season that starts in January.
This claim is also supported by the fact that at this point of the year, no trailers, spots, character designs or announcements of any kind have been released promoting the season, as it is happening with the rest of animes that will be airing during the Fall lineup.
The reasons behind the delay are still unknown. As revealed by Wit Studio's president George Wada on the Sakura-Con 2016, the season is currently under production, so is very likely that given the short staff list of the company and the other projects they have been recently working on (including the original series Kabaneri of the Iron Fortress, which uses much of the staff of AoT), they were unable to meet the original schedule. As soon as we have more information we will let you know.
UPDATE: This is the transcription of the audio interview included on Mision Tokyo's blog post. It is just a 1:20 minutes long fragment of the full interview, which will be uploaded by Mision Tokyo to their ivoox account this week.
The part where Manu Guerrero talks about their intentions to simulcast the season is not included in the fragment, but in the text of the blog post, which reads as follows: "Although everything suggested that it would be one of the strongest releases for the end of the year, [and] in fact it was expected its release for October, it seems that it will finally be delayed and we will have to wait a little longer to see new episodes. This is what has been informed by Kodansha, the owner of the license, to Selecta Visión, publisher and distributor of the anime in Spain, in talks to broadcast the series in simulcast once it is released in Japan."
And the fragment of the interview:
MT: And, about Attack on Titan, the second season also...
SV: Well, Kodansha... Kodansha has confirmed us that it will not be [released] this 2016, and that it will be for 2017.
SV: We hope it will be... of course, we, those who look forward for Attack on Titan, hope it will be [released on] January 2017, coinciding with the new anime season in Japan, we hope it will be so.
MT: Everybody thought it would be in October of this year...
SV: Many people thought it would be in October of this year, but as you know, the announcements of the new seasons are already being made, and previous to the announcements they would have released character designs, the studio would have made some kind of, let's say, press release, they would have released spots, trailers... and if at this stage there is nothing, that is because it will not be for the October season.
UPDATE JULY 3, 2016: Wit Studio has confirmed that Season 2 will be premiering in Spring 2017.