Apparently, Disillusioned Adventurers Will Save the World, Vol. 4 (light novel) — Shinta Fuji

THE DEEPEST SHADOWS CAN HIDE BEHIND THE BRIGHTEST LIGHTS.

After a fierce battle, Nick and the others succeeded in crushing the demon-god worshipper White Mask. The guild is extremely happy with their performance, so the Survivors are entrusted with Nick's dream job: guarding an idol concert! But when they discover counterfeit tickets -- and a connection to the demon god's adherents -- they start to doubt that this is a normal escort mission. And that's all before Sun Knight captain Alice shares her suspicions about Nick's former party member Garos, shaking him to the core. Karan is there to lend her support, but her feelings for Nick seem to be changing...