Error while running setup for entity prototype "ultimate-transport-belt-loader" (loader): next_upgrade target (fast-loader) must have the same grid alignment (bounding boxes aren't compatible or tile_width/tile_height aren't compatible).
I can't find a mistake to fix by myself.

Ok, I had an idea after posting this.
Seems like "Loader Redux" (or "Miniloader") are using the same names. I had both enabled.
Make it incompatible maybe?

Something tried to give the t5 bob's compact loader a next_upgrade of the t2 vanilla loader.. that's incredibly strange. It seems to be Loader Redux but looking at the code I can't understand why. I'll dig into this!

Ohhhh I see what's happening - Loader Redux overrides what used to be the t1 vanilla loader early on in the game loading, before Deadlock Integrations loads anything at all (and it copies the base for all of its loaders from that one), with a loader that has a next_upgrade. Because it's the last tier, I don't override its upgrade attribute. Should be an easy fix!

The more robust fix involved an update to the base mod, grab 2.0.8 of https://mods.factorio.com/mod/deadlock-beltboxes-loaders and that should get things working correctly.