I have 10 Mk1 cannons around nauvis and I am currently on vulcanus (no cannons).
Loading the save after updating the mod gives the following error

The mod Orbital Ion Cannon (Space Edition) (3.8.0) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event Kux-OrbitalIonCannon::on_nth_tick(60)
__Kux-OrbitalIonCannon__/modules/gui.lua:306: attempt to index field '?' (a nil value)
stack traceback:
    __Kux-OrbitalIonCannon__/modules/gui.lua:306: in function 'update_GUI'
    __Kux-OrbitalIonCannon__/control.lua:132: in function <__Kux-OrbitalIonCannon__/control.lua:109>

sorry, the code has been changed in the meantime, so this stacktrace no longer returns the correct lines to localize the error
please try 3.8.1 and report it again if the error still occurs .

I tried messing around with the code a bit, but the error didnt make much sense given what I was seeing, so I got the idea that maybe another ui mod was messing with it, and looks like this was indeed the case.
Loading the save with https://mods.factorio.com/mod/GUI_Unifyer_Unified disabled stopped the error from happening.
I'm guessing I should report this to the other mod author for a compat fix?

I'm guessing I should report this to the other mod author for a compat fix?

yes, please.
But I'll also test it with this