The school grounds were not purified and that plays a role in a number of problems
Nanase's father and grandfather lied about it for PR but not wanting the cost of actually doing so to solve a supposedly false issue, one that is very much real. It wouldn't be the first time a tragedy happened as a result of corner-cutting measures. The 19XX Landslide has its roots in both real and supernatural
Sakura warns Nanase about her family cutting down too many trees. This has two reasons: One a forest in the area is said to have spirits that will spirit away anyone that steps into it and trees help keep soil in place. The severity of the landslide and the suspicious timing hint at a supernatural boost from the enraged spirits. And if the unpurified school grounds WMG is right, the location if not the town may have become particularly cursed by 20XX, particularly since there's no sign of anyone from Sakura's priest/miko family in sight that could help.- Seemingly jossed in Chapter 21. Sakura actually states that the landslide was caused solely by deforestation, with the curse being unrelated to it.
Korone is the spirit of someone who disappeared at the station
Korone is surrounded by static in her appearances, which given Shino and Nanase means that something's up with her. She's seen near the station, where a string of disappearances of Aogami High students took place, and it's implied that the students were abducted and sacrificed to the town's god. It's possible that Korone was one of those victims, and her spirit came back to get vengeance and/or ensure others don't suffer the same fate that she did.- ...kinda confirmed? The entire situation with the 20XX cast, Akane included, is really hard to decipher, but she was indeed involved with the train station disappearances.
Shino is the 20XX New Transfer Student and a case of Literal Split Personality with some for of time displacement/reincarnation involved - the nebulous half of her that's mainly seen in Episode 3, the game and Chapters 2 to 5 and 8 (as of writing) wants to become a whole entity again
The key visual animatic is the first indicator - the only time that Shino actually appears in the 20XX uniform is during her Jump Scare in Episode 3, and that may be just to directly mock her split self, if the notion of dreams is one of the project's underpinning core elements.The second indicator for this is seen in the manga's individual chapter end credits: Chapters 1 through 5 list Shino and the 20XX girls who appeared, as well as Honoka, Nanase, Shiina and Yae for Chapter 5. Chapter 6 and 7's end credits then inexplicably list the 19XX Transfer Student, albeit without a listed actress to preserve the mystery. However, Shino has been listed in every credits since Chapter 1, where her actress, Sora, acted as a Character Narrator of sorts, even though Shino didn't actually appear in the manga until the very end of Chapter 2. Even unspoken roles (sans flashbacks according to Chapter 8) are credited, as seen in Chapters 2 (Saki) and 5 (all but Shino). Ultimately, the reason the Transfer Student for 20XX doesn't have a listed actress during her appearances, as opposed to her Past counterpart, is because Sora is set to play the nebulous Shino and the Present Transfer Student, thus avoiding the actress being listed in the credits twice and giving away a later reveal.- Confirmed in Chapter 15.
The setting of hololive ERROR is a Split Timelines Plot Mystery Visual Novel wherein the data is corrupted and hasn't been ironed out through quality assurance/beta-testing.
Each of the girls' bios (Shino, Inari to Saki and Nanase to Yae) as seen on the characters page read out like a bio as to what you would see for a character in a Dating Sim Visual Novel, and the ambiguous event/earthquake and landslide, for example, are each the supernatural elements within the story that must be resolved while working towards an ending with any of the eleven girls, and such supernatural elements are tied together by a common cause. However, Nanase's data is the most buggy, and trying to pursue her "route" makes the buggy coding propagate into other elements up and down the game, such as Shino appearing in the Present when she shouldn't; the classroom sequence from the game demo and Chapter 5 is supposed to be a nightmare scene seen by one or both Transfer Students, or, alternatively, they're meant to see respective versions of the scene but they've been fused together for the 20XX Transfer Student. In turn, this renders the February 20XX glitchy and repetitive for a good chunk of it, breaking a scale of time as seen in Chapters 9 and 10. The girls of 20XX are actually the souls of sacrificed children, and they're being used as Shino's friends for her perfect world fantasy.
The background lore of the series is that Human Sacrifice of children were very common, and were used to appease to the God of Aogami to stop the landslides. Considering what happened to Shino, it's possible the girls in 20XX are the souls of those children who were sacrificed to the Aogami god, and ended up in Shino's perfect world fantasy until the God decides to stop with the masquerade and tell her the truth. The second Shino is actually the God of Aogami.
We see Shino praying to the god of Aogami so she can have friends, and the god seemed to have delivered. Until that fateful night when the landslide killed everyone except Sakura and the Transfer Student. It's possible the god took Shino's form after she prayed to it, and when she encounters her in her blue-eyed yet condescending form, the god decided to stop having fun with Shino's fantasy and let her truly realize that she's in a fake world. The art room student from the game is a Paradox Person.
Yes, we all know the game's creators said they didn't make the blood painting's creator Mari because they didn't want to bring on-screen harm to a hololive talent. That's the Doylist explanation, and for something that seems pretty significant too. Her version of the rumor is also a lot more violent than Mari's, and Shino states during the climax of the Accepted route that there's some sort of error in this version of the worldnote . Shino has always shown some subconscious control of the Perfect World, so it could be that this is a stand-in created to be the blood painting's creator... but she would've then had to disappear and make way for Mari once Shino realized the truth about the painting. However, just like the student registry in the train and protagonist Shino's memories, she persisted through the Perfect World's constant self-alteration and her own possibly-attempted erasure. She is/could be the error of the new Aogami.