Ultima: The Maze of Schwarzschild

Ultima: The Maze of Schwarzschild ウルテイマ ～ シュワルツシルドの迷宮 (Urutima ~ Shuwarutsushirudo no Meikyuu) is an Ultima-inspired  manga created by artist Seiji Tanaka. It was published on November 15, 1991 by JICC and originally sold for ¥980.

Synopsis
The manga is a direct continuation of the "Exodus" manga. Aida and Lennon have resurfaced in Genji's outer-space "homeworld," under new identities as caretakers of a space station. But so have Mondain and Minax -  as scientists who have discovered the remnants of Exodus trapped within an asteroid and seek a way to unseal the dark god.

The hero Genji is shown on the front cover, with Aida, Lennon and Bigelow on the back.

This here is destilled from RACapowski's page, who actually does own the manga but currently doesn't have the time to share the contents:

The story actually takes place in Genji's world, not Sosaria. Aida and Lannon reside on a space station, Genji is the result of cloning and Bigelow almost doesn't appear at all. Mondain and Minax (as scientists) find the remains of Exodus inside an asteroid and the heros try to prevents its resurrection, resulting in a battle with motorycyles, mechas and in the end in space. This is littlered with flashbacks where the characters life on Genji's world in the past is shown, such as Genji and Aida in Vietnam, with no continuity to the Exodus manga.

The manga therefore has next to nothing to do with the Ultima universe. As for the title, while a black hole appears late in the manga, explaining the Schwarzschild as a black hole has a "Schwarzschild Radius", there is no labyrinth of any kind.