Which is the same message I previously missed in the Unreal Build Tool Log.txt arf… It clearly says it’s attempting to copy some files from some location to another. It involves a project called “Datasmith” and if I look in the solution explorer of visual studio there is a project in programs/Datasmith/DatasmithSDK. I try to rebuild this specific project instead of the whole solution (the later is excruciating slow). Bingo, the error shows up again, well at least now I can quickly recompile this part and try various things. First I make sure files and folders are no read only. Recompiles … Nope. Then after some random search I figure out where this copy is specified: it’s in the build configuration file DatasmithSDK.Target.cs it’s calling the command xcopy and by the looks of it does not wrap the file paths with extra quotes and so it does not correctly parse file paths containing spaces! This is a mistakes I’ve made myself countless times aha! Hooray 
To summarize with this we’ve learned: how to find the build scripts of UE, where are the logs of Unreal Build Tool, overall get familiar with part of the folder structure of UE’s (modules and separate programs build tools) and pinpoint and only compile where the error comes from. But above all we’ve learned that we should better read the details of error messages… I could have went straight to where xcopy was just by looking up the string “Runtime\Datasmith\DatasmithCore\Public” if I had better read the error log and a bit of luck.