Current forge for 1.15.2 crashes on launch with only Bookshelf-1.15.2-5.3.9.jar in mods folder.
crash-2020-04-09_01.28.05-client.txt

Sorry for the earlier edit, my initial report and attached log was for 5.3.9, BUT 5.2.5 seems to have the same issue.
crash-2020-04-09_01.33.32-client.txt

5.1.4 launches successfully.

You're using an outdated forge version.

No I'm not.

3.1.0 is old, this mod requires new forge features.

wut.
31.1.0 is less than two weeks old and is still the current recommended version. How is that old?

The latest version is 31.1.41. The number at the end goes up every time forge updates. You are 41 versions behind. Furthermore, 31.1.0 came out on February 3rd 2020, which is 2 months and 6 days ago.