I am still seeing errors with Storage Tank Mk 2 enabled. I am getting the message Failed to load mods: Error in assignID: entity with name 'mini-tank-5' does not exist. Source: mini-tank-4(storage-tank).
Not sure if Optera did an update between the other issue with the mod and now but since I like the mini-furnaces better than 100k tanks, guess the Storage Tank Mk 2 gets disabled. :)

Hm, currently testing with Factorio 1.1.6, using just this mod and Optera's Storage Tanks. I was using both in my latest playthrough as well, and no issues there.

You might have another big mod in that pack that might be interfering? Bobs, Angels, or something similar? I don't believe they did anything except update the version number, so it must be something more specific?

I have same error if enable "Bob's logistics mod" ver 1.1.3

Ahh... I see the issue. If Optera Tanks is detected with Bobs Logistics, the Optera code messes up the logic regarding the "next_upgrade" target, so it accidentally overshoots the final tier in Bob Logistics and attempts to reference a non-existent tier 5. This bug would actually happen if Optera Tanks is detected with any other mod that adds additional tiers of storage tanks... wonder if that might explain some of the random errors I'd been unable to track down.

Thank you very much for providing the modlist, wouldn't have been able to debug this issue without it. Although it's a little weird to run both Optera Tanks and Bob Logistics, considering they add different "tier 2" storage tanks, with the Optera storage tank being much bigger and cheaper than the Bob Logistics storage tanks. If you run all 3 of these mods, the mini tanks will have stats based on the Bob Logistics tanks, not the Optera tank.

Either way, the load error should be fixed as of v8.0.1