From this new versions Library 4.2.18 290, to access your Dropbox cloud account you need to login to it first in this way :

• from the Library page clic onto the three dots in the upper bar on the right
• the PadMu account window will appear, clic onto the Dropbox login button
• complete the login process (it is a just allow if you were already logged in before the update)
• at this point the button will show "Logout"
• clic on cancel in the window to go back to the LIbrary
• click on add files icon on the top, you will see the dropbox icon back as used to be before
• you are done !