The problem seems to be the structural fuselarge. I also was unable to use that as a rootpart for my welds. It was the only part that gave me the issue so far.
Just wondering. What is the effect of 'breaking a mod'. As i have tons of mods installed it takes almost 4 minutes to load the game. Should i build a massive rocket do i have to reload after every weld or only when it's done and before the launch?