Crash report here.

This happens intermittently, and never twice in a row (the world loads fine after launching MC again). As far as I can tell, it isn't related to whether or not I'm actually standing within range of a Solegnoila, nor if an item within range of a Solegnolia would be affected by my magnet.

Sorry if this is a little vague. I've been trying to figure out more about this, but I still can't reproduce it on demand. The last time I saw it before today was one week (fifteen or so Minecraft launches) ago.

Botania r1.8-248
Forge 10.13.4.1558

Can you reproduce (or try to) this with just botania installed? Also, are there any chunkloaders nearby?

On a side note, I like how you were ~150 blocks above sea level when the crash happened

There are no chunkloaders anywhere in the world where this happened. I have tried to produce it with just Botania, but as I mentioned, this happens very rarely.

(ATG volcano is the reason I'm that high up)

a CME? o.o
that shouldn't happen... someone's calling things from threads.

It can be solved by making a new ArrayList out of existingFlowers, but it shouldn't exist in the first place...

yes, cme is possible in this case. This is because the client and server threads were accessing a static field at the same time (server is reading it, client probably added or deleted something), which fits with the description of this being an SP world. It's a common siding mistake and I would've suggested removing the map from the clientside, but a map per side is needed in this case - for the ring of magnetization to work and look nice the logic needs to run on both sides.

The Weakhashmap needs to be threadlocal or split into two, one for each side.