I've unfortunately found that if you place a rune holder at Z 0 it causes a divide by 0 error, its really easy to recreate and it's because Math.floorMod throws an exception if the divisor is 0, which from looking at the code is current.getZ().

Here is the important bits from the stack trace:
-- Head --
Thread: Render thread
Stacktrace:
at java.lang.Math.floorDiv(Unknown Source) ~[?:1.8.0_161] {}
at java.lang.Math.floorMod(Unknown Source) ~[?:1.8.0_161] {}
at mythicbotany.rune.RenderRuneHolder.render(RenderRuneHolder.java:41) ~[?:1.16.4-1.3.10] {re:classloading}