Failed to load mods: Error in assignID: entity with name 'linked-5d-underground-belt-04' does not exist.
Source:linked-express-underground-belt (linked-belt).
Mods to be disabled:
• LinkedBelts (0.1.4)
• Kux-SlimInserters (3.0.26)

Removed SlimInserters but still the same problem

Can you provide the full list of enabled mods? I can see that you are using 5Dim's transport at least, but I've tested it before and didn't get this error.

Mod list is very long, so shared with this link instead. Also when I disabled the linked belts, works fine

https://codeshare.io/R7qQkL

Hmm. It could be caused by combining deep-underground-belt-2 and 5dim_transport (together with LinkedBelts). Is there a reason that you use both? 5Dim's transport already adds more underground belts.

I tested enabling mods one by one and found out that The 5Dim's transport caused this error. I don't know why yours works. I tried uninstalling and reinstalling, still doesn't work

It seems that deep-underground-belt-2 + 5dim_transport + LinkedBelts (and with dependencies base, 5dim_core, bobinserters) does not give the error, do you know if any of the mods add more underground belts?

Space Age adds the turbo belt, since I haven't bought the DLC yet, can you test LinkedBelts + space-age + 5dim_transport for me?

I'll try adding more mods in the mean time to see if I can get the issue.

Disabling Space Age seemed to fix the issue.

Does this issue still happen with space-age + 5dim_transport?

No, disabiling the linked belts while leaving space age and 5dim's transport still works

No, I mean, with 5dim_transport + space-age, do you have two belts with a speed of 60 i/s? Then I regard it as a compatibility issue between them (unless there is a good reason to have two belts with the same speed).

Things like this can break LinkedBelts, as it skips belts that have the same speed as other belts, but doesn't check if the skipped belt is required for upgrades. So I think what happens is that linked-5d-underground-belt-04 is skipped as it would have the same speed as linked-turbo-underground-belt which has already been added, but the linked-express-underground-belt still upgrades to linked-5d-underground-belt-04 (because this is how the express underground belt upgrades with 5dim), which causes an error when Factorio checks the names at the end.

Yes, there is two belts with same speed, the turbo transport belt from space age DLC and the transport belt mk4

Since I don't think it's possible to solve this issue on the LinkedBelts side (I still encourage you to report the SA+5Dim incompatibility to the developers of 5Dim if they aren't aware of it yet), I added a startup setting that will stop LinkedBelts referring to entities it doesn't create.

So if you enable that startup setting (before adding either SA or 5Dim's Transport, else it will crash) you should be able to play with SA+5Dim's Transport+LinkedBelts.
Let me know if there are any problems.

Do note that if 5Dim's is updated to remove the duplicate underground belt, or if they are loaded in a different order, the linked belt based on it might disappear, so enable the setting at your own risk.