So testing to publish your map worked for me. 

It is an issue with your local Steam. Sadly Steam  is unhelpful with their k_EResultFail as it's their generic "There is an issue" message.

So it's not firewall, nor their EULA.

1) Have you got a lot of save files / workshop subscription ? (If so try to delete a bunch and see if it helps)

2) Does the workshop works for other games by any chance ?

3) If the firewall did not help, there is still a chance that your router is blocking the traffic, if you have any chance of uploading a map outside of your normal environment and it works, that would point to the router. (like this thread for example)

I'm running out of other idea to find the culprit. But the map uploading fine on my side shows that something somewhere in your local setup prevents it ^^' We just need to find out what is preventing the upload

Hi Hiding,

I suspect an issue between Steam and your firewall, but can you upload the actual map you're trying to publish  (the _edit version)? How to: Locate your save files

Or is it any map ? (If it is any map, as said it might be your firewall /antivirus, maybe try to disable it temporarily and try again ?)

- I think after spawning 3rd Thunderling and trying to add them to rally group 1 along with the Behemoth, their rally (both - for Behemoth and Thunderlings) buttons on the left side of the screen disappeared,

This is intended, once there is nothing to control in the default group (Thunderling/Titan) , the group disappear. (As there is nothing to do with it). It reappears if you remove those units from specific groups