Moved to the Workshop. I haven't done maps, but I'm going to see if I can look up anything that might help.

In the meantime, also check:

https://community.dawnofwar.com/categories/mods and
https://community.dawnofwar.com/categories/community-creations

For any existing resources or guides.

I was following this guide http://modding.dawnofwar.com/multiplayer-scenario-tutorial and a video on youtube which describes the same process. I even went as far as to search if there was a similar problem with CoH2 world builder because it's very much alike, found nothing so far.

The problem is that there's not much that can go wrong, just place 3-6 objects (depending on game mode) for each player, assign owner, calculate voronoi, done. Except something did go wrong and I have no idea what exactly.

It almost sounds like it can't find a proper reference for something. Did you set up the tool paths correctly? Under "Configuring The Mod Builder":

http://modding.dawnofwar.com/mod-builder

I was happy to find out that I don't even have Mod Builder installed, I thought World Builder is all there is, alas it's still not working. Anyways here's a screenshot of the paths:
https://imgur.com/4UgP20S

The guide in your link doesn't say to fill Tools and Signature paths and I have the former filled but it doesn't work either way so just ignore it. Also the "..." button crashes Mod Tools for some reason so I had to copy paste paths. Also had to install java and flex sdk, the latter was just an archive with a bunch of files and no installer, I found it weird.

The whole situation stinks, I'm happy to admit that I'm stupid but the guides are pretty straightforward. I was playing with world builder for a while now, I just haven't saved anything. I've figured out what kind of sets can be build using existing objects, ground textures etc how light settings work, I've made a map layout in graphic editor. I was just a couple of days away from making a decent looking map and it turns out the basic stuff is not working. It just sucks, people are making mods to keep the game alive and I though maybe I can do something too, a map or two for example as I have experience with making maps for SC2 but nope, something is not working and there are no error logs to find out what.

Found a couple of strings in debug window as I create the map:
FILESYSTEM -- filepath failure, missing alias 'GENERIC:scenarios\'
FILESYSTEM -- filepath failure, missing alias 'GENERIC:scenarios\*.scenario'
FILESYSTEM -- filepath failure, missing alias 'GENERIC:scenarios\*.scenref'

And another string when I try to save:

ToolFramework: Saving document: 'C:\Users\(unintelligible symbols)\Documents\My Games\Dawn of War III\modssource\scenarios\NewScenario.scenario'

Also if I try to open file (although there is nothing saved to open) I get a notice error saying "Cannot open location with this program. Try again using different location" (rough translation).

Also found an old company of heroes thread, in the end it says sommething about world builder not saving https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=175370459

World Builder Not saving
Renamed "pozovatelya name" in English, and save path chose this:

"D: \ SteamLibrary \ steamapps \ common \ Company of Heroes Relaunch \ WW2 \ Data \ Scenarios \ MP"

save success!

No clue what the hell that supposed to mean. My windows user profile is not written in english, maybe it has something to do with it.