Room temperature as far as most baking/pastry applications go is between 68-72 degrees (+/- 2 degrees per some texts I've seen). If your butter is too soft aka somewhat melted, you will end up with a soupy mess. At the above temps butter is soft but still plastic in nature meaning it holds its shape and can be bent without it breaking. If you poked it with a finger it would give and leave an indentation. Think of it this way, butter in solid state has a crystalline structure; as it warms up, the solid crystalline structure converts to liquid oily structure, once it goes there, you can't get that solid configuration back. If you placed melted butter in the fridge it would get solid but it won't go back to the solid it was when you bought it.
Just like when you cream butter and sugar for a cake, you're often told that if the butter is too soft or melted it's ruined for creaming. This is because melty butter no longer has the plastic structure for the sugar crystals to cut into to create the minute air pockets that are essential for the creaming method. It's sort of the same with your smbc. If the butter is melted or warmed to a certain point, it's ruined for that application (you could still use it for applications that call for melted butter like some types of cookies for example).
Shall I assume that your "room temperature butter" left overnight was actually warmer than implied room temp? In the previous attempts you brought your butter out right before you started and it was soft enough to give a successful result, that leads me to believe that leaving your butter out in similar temps overnight gave you butter that was too warm hence the soup. If the first method worked for you why not stick with it?
If your butter is too cold, you would end up with a chunky curdled mixture. This is typically not a bad thing because with continued mixing (as newbies are often advised with smbc) the mixture warms up gradually allowing the butter to warm up and giving the emulsion a chance to come together. When your butter is too warm/melted, there's little structure to work with. In some cases you may be able to salvage the soup by cooling it briefly before mixing again but often that results in BC that's less stable than if your butter was the right temp in the first place.