As NBT said, if you go inside the house and place another wooden block on the inner layer, then it converts it into a roof. It's because it needs something to affixiate to to become a roof, like stairs rather than a diagonal line.
I don't know if it's related but this issue never appeared for me until we tried to dispose of stumps on the server. When we got rid of 3 stumps on the server, this line 'Object reference not set to an instance of an object' appeared three times in the server log and eventually kept reproducing and doing so until the server disconnected.
Mine at home took about 10seconds (actually less, but I am rounding up) to start server on my brand new desktop at home. i am presuming it is a RAM issue as my home computer is overpowered in that department. A work server would be using ram for the various other stuff operating on server whereas my home computer is doing nothing but what I tell it.
(just double checked and experience still was less than 10s)