@iska Okay, reading the license header:
RPG Paper Maker engine is under LGPL-3 license.

Commercial license for commercial use of your games:
https://creativecommons.org/licenses/by-nc/4.0/.

The author is committing doublespeak, saying "this is free software" and "this is proprietary software" at the same time.

The reason why the author licenses under LGPLv3 is because (unless you pay for an exception), qt5 is licensed under GPLv2 or GPLv3 or LGPLv3 and your software needs to be under a compatible license.

Even so, CC-BY-NC 4.0 programs can use LGPLv3 libraries provided the LGPLv3 library and CC-BY-NC 4.0 parts are separate and are dynamically linked (the CC-BY-NC 4.0 doesn't do things like forbid reverse engineering totally. There's also other ways to combine, but that's out of scope really).


The LGPLv3 is the GPLv3, but with certain extra exceptions - it gives permission to convey modified versions under the LGPLv3 OR under the GPLv3 no matter what: "2. Conveying Modified Versions."

As the GPLv3 notes:
7. Additional Terms.
...
All other non-permissive additional terms are considered "further
restrictions" within the meaning of section 10. If the Program as
you received it, or any part of it, contains a notice stating that
it is governed by this License along with a term that is a further
restriction, you may remove that term.
...
You can just delete the "Commercial license for commercial use of your games: https://creativecommons.org/licenses/by-nc/4.0/." part from the license headers, or upgrade to GPLv3 and then do the deletion if you aren't confident.


There are some other files without a license header or a marked license though:
Editor/Ressources/*
Editor/Shaders/*
etc.

As I don't see a LICENSE file, I believe "RPG Paper Maker is a game making engine, free [sic] for non commercial use" in README.md is a restriction that applies to the unmarked files.


Pretty much, the developer(s) of that game engine seem to be an incredibly nasty sort, so I would recommend using a different game engine instead.


If you still want to use it, you'd best go and start with the older version and remove the CC-BY-NC 4.0 headers and replace the files that seem to be licensed only under the CC-BY-NC 4.0.