The crash occurred when I clicked on an entry in the Astral Tome.
I was connected to a dedicated server at the time, however only the client crashed.

It does not seem to matter which entry is clicked, however sometimes, for no apparent reason, clicking does not cause a crash.
When using the Tome in a singleplayer session, there seems to be no issue.

I am using mod version 1.15.2-1.12.4 and forge version 31.2.33.

The full crashlog can be found here