My reply might contain a shit ton of spoilers but that's the only way to help you.

In this game, there are 4 girls as you know:

Sayori, Natsuki, Yuri and Monika.

Monika becomes self aware as the game progresses and begins to alter the other girl's stats to make you, the main character, avert your eyes to her... yeah, to 'Just Monika'. This explains why she is always so calm and cheerful and as the story progresses the other girls begin acting out of place and tend to harm themselves. Whenever you'll neglect one specific girl out the 4 except Monika, they will eventually become sentient and at the end, kill themselves... As Monika is controlling every fucking thing, your choices that you make have literally the 'least' effect on the girls. As a character dies in the game, Monika, now the mastermind of the game, deletes their character files which means they are no longer existing, and that is the way the story progresses... There's no need to save anyone. Sayori will die and so will the others, until Monika is the only one left.  That's the way it's meant to be played. There's simply no point in trying to save Sayori or anyone. The player needs to tweak with the character files (the .chr files in the DDLC directory) and at the end when Monika is the last one alive, you have to delete her character file to see the true ending of the game which contains references to a future game coming out later this year in 2018. You might as well see random files or pictures appearing in your game files as you progress through the game... search online to find tons of scary eastereggs the game contains.

Sato Kun

The "Exception Occurred" to me too in this same scenario. I think it is just one of the story endings.

I played the whole game in ubuntu xenial on x86_64 cpu (running ./DDLC.sh), with no issues at all but this, which is showing as no-turning-back.

Aside from the exception message itself, never nice to see, the game suffered from errors from that time on. The main menu entries became corrupted, so I had to quit the game. Launched the game once again, and I saw the same font corruption, further the program deleted ALL my saved games AND no new games can be started because the engine says one of the character files is corrupted (can't remember the exact warning).

I would like to help developers but I currently don't know how to attach the traceback text file created by the game.

Ha, I reinstalled the game too when I first got to that part. I didn't lose anything though, I just missed out on some lines of dialogue at first, but unless you delete the firstrun file and choose to delete your save, your progress is still all there.