Had to create and account just to post here, there is a fix. I was having the exact same issue, black or in some cases dark gray transparent bars, they were even the same shape as yours.
Someone above mentioned the Advanced camera tool (which installs with sketchup 2017) and even though i had not used it to create any cameras, all of my scene tabs would export a 2D jpeg with those bars on them, all except the default scene 1 tab! Weird huh.
Anyway, go to your scene tabs and then go to Tools -> Advanced Camera Tools -> Reset Camera . Your scene view will adjust a bit, go ahead and just zoom back to where you want it to be and your exports will work fine. Do this to each scene tab, save and you are good to go.
No idea why this is happening, but props to john_drivenupthewall for setting me in the right direction.