Chaia The same thing happens to me. I purchased a new pc and tried running a fresh copy of the desktop version (r157). However, I had no problems opening Dropbox on my old pc, with the same version (r157). That confuses me.
Ashley has addressed this in a recently closed bug report here... github.com/Scirra/Construct-3-bugs/issues/3147
Until then, the way I got around it was to QUICKLY close the Dropbox api popup as soon as it opens. That will prevent the crash and you can then select another service (ie. OneDrive or Google Drive) which do work, btw.
Hope that helps.