In the past months, my logo was located in the upper right corner (it was fine there). Now, for some reason, it moved to the middle and I don´t like how does it look. Is this some kind of bug due to upgrades on cloud version or how can I relocate to where it was? I am using plain theme.
So you are not, in fact using the Plain theme. You will have to modify your custom theme. Begin with the change outlined here. After that, you are on your own.