Excellent! Yes, you are right.
Disabling AV and firewall is one of the things I usually try when something doesn't work, but it never crossed my mind it would stop a compiler or linker working.
I use Commodo firewall and it was its sandbox feature causing it. Pspawn.exe, I think. Made that trusted and it works fine with the AV/firewall turned back on.
Also, it's worth noting that once all the output files in the Release directory existed, the compile and/or build process would carry on working even without the fix, as above.