Let me try look it up. I think one problem is that the number of civs increases, leading to your file being invalidated.

Here is some quote on some discord servers

This is not the solution. The solution is as described in the error text
[10:48 AM]
Which I was hoping would make it clear what needs to be done
For example if you have 10 civs in your .dat, also include civilizations.json that declares those 10 civs.
That way you don't have to add new ones everytime if you don't want to.
And it will avoid crashing the game when going Random Civ during game setup.

Always include civilizations.json
Never make a data mod without it. (edited)

@StepS
This is not the solution. The solution is as described in the error text

My mod basically introduces an upgrade above Elite for UUs, will make use of scenario units and renames the original scenario units, so every time when I load it into a scenario and don't see units like named "Amazon Warrior Old" at the top, it lets me know it's not loading.

I've tried various things. Without taking my created RESERVEDs into consideration, my mod's Techs IDs were 981-1050, Effects IDs were 981-1050 (they match!), and my Units IDs were 1902-1992 - the new dat file has "RESERVED/Nonexistent" for these, so I could copy/paste them right in without having to relink tech and effects.

The very first thing I did was accidentally use insert copies on the Units which shifted all the IDs up one, which messed up the "tech trees" and caused a CTD in-game when I clicked on the tech tree, the mod was working at the time, though. So I went back fixed this and pasted them properly on a fresh dat.

Didn't work. Copied the Units ONLY and pasted them at the end after "2381 - GOAT". Didn't work. Included the civilizations.json, didn't work. Did this before anyway. Tried the PUP version as well.

Appreciate the effort, I'll just wait until the civilizations are out. Seems like a bad idea to be doing this stuff right now anyway.

Sorry to double post, but importing my local mod seems to cause it to work. It will then show a "loading data mod" box when selecting the data mod in the editor, and load correctly. Otherwise, booting the game and going straight into the editor does not load it, which it used to - without doing anything extra.

I don't know if that's normal or not but I'll take it.

Devs has intentionally destroyed modding support since the first post-RoR update, basically an year an a half since mods were cut. More than two decades with the community keeping this game alive with mods that relied on editing empires2_x2_p1.dat entirely out of passion and fun purposes, now completely gone just because yes.

The worst part is that they don't even give us a proper way of using a custom dataset since very damn time we open the game, we have to load it manually with endless annoying popups of confirmation, and it keeps resetting once we leave the map unlike other mods.

No wonder why AoE2DE has so little active modders providing Total Conversions and other cool stuff. With every update devs breaks something and don't bother fixing it, making people prefer AoE2HD since it's long dead for the better.

i've had this same problem as well. i've been modding aoe 2 since 2008, and aoe 2 de just.. has so much about it that.. i dont really know how else to put it besides, hates modding. The implementation of the modding system on microsofts end, instead of just using steam workshop, is just horrendously clunky to interact with. Hardly any mods are still updated besides competitive multiplayer quality of life mods that should just be toggle options in the menu by now.

even just making a simple balance data mod to play with friends in custom lobbies is tedious beyond measure. custom scenarios are automatically downloaded, but when its a data set mod, it isnt? so now we always end up in the sitation where my friend downlaods my mod, enables it, i active the data set in custom multiplayer lobby, and it always says, without fail, that his version is out of date and we cant play.

like man. what is even up..

I have problems in loading a created data mod that worked before this big change in the new version 128xxx...i had no problems before...just copy/paste what i made on new version of data mod and save it...that's all i had to do before...now doing this is useless...always accompanied by "Error: Problem Loading Data Mod"...i haven't added new things...WHAT DID THEY CHANGE that made mods useless?...am i missing something?
I want to mention that i'm not linked to xbox nor will i want it too...it worked loading data mods without needing to have xbox account.
PS: I would attach a photo if it were possible..

Edited: The error above has been solved by clicking "Import Local Mods" when going to My Mods tab