I downloaded and installed it. The indicator was working, but it was the other symbol (not the new one with non-working context menu). There are also no icons in caja marking the sync state.
I stopped Dropbox and started it with the mentioned command:
christophe@cXPSmate:~$ XDG_CURRENT_DESKTOP=Unity dropbox start -i
Starting Dropbox...dropbox: locating interpreter
dropbox: logging to /tmp/dropbox-antifreeze-SkbDEf
dropbox: initializing
dropbox: running python 2.7.11
dropbox: setting program path '/home/christophe/.dropbox-dist/dropbox-lnx.x86_64-38.3.23/dropbox'
dropbox: setting home path '/home/christophe/.dropbox-dist/dropbox-lnx.x86_64-38.3.23'
dropbox: setting python path '/home/christophe/.dropbox-dist/dropbox-lnx.x86_64-38.3.23:/home/christophe/.dropbox-dist/dropbox-lnx.x86_64-38.3.23/python-packages-27.zip'
dropbox: python initialized
dropbox: running dropbox
dropbox: setting args
dropbox: applying overrides
dropbox: running main script
dropbox: load fq extension '/home/christophe/.dropbox-dist/dropbox-lnx.x86_64-38.3.23/cryptography.hazmat.bindings._constant_time.so'
dropbox: load fq extension '/home/christophe/.dropbox-dist/dropbox-lnx.x86_64-38.3.23/cryptography.hazmat.bindings._openssl.so'
dropbox: load fq extension '/home/christophe/.dropbox-dist/dropbox-lnx.x86_64-38.3.23/cryptography.hazmat.bindings._padding.so'
dropbox: load fq extension '/home/christophe/.dropbox-dist/dropbox-lnx.x86_64-38.3.23/psutil._psutil_linux.so'
dropbox: load fq extension '/home/christophe/.dropbox-dist/dropbox-lnx.x86_64-38.3.23/psutil._psutil_posix.so'
dropbox: load fq extension '/home/christophe/.dropbox-dist/dropbox-lnx.x86_64-38.3.23/linuxffi.pthread._linuxffi_pthread.so'
dropbox: load fq extension '/home/christophe/.dropbox-dist/dropbox-lnx.x86_64-38.3.23/tornado.speedups.so'
dropbox: load fq extension '/home/christophe/.dropbox-dist/dropbox-lnx.x86_64-38.3.23/cpuid.compiled._cpuid.so'
dropbox: load fq extension '/home/christophe/.dropbox-dist/dropbox-lnx.x86_64-38.3.23/linuxffi.resolv.compiled._linuxffi_resolv.so'
dropbox: load fq extension '/home/christophe/.dropbox-dist/dropbox-lnx.x86_64-38.3.23/posixffi.libc._posixffi_libc.so'
dropbox: load fq extension '/home/christophe/.dropbox-dist/dropbox-lnx.x86_64-38.3.23/librsyncffi.compiled._librsyncffi.so'
dropbox: load fq extension '/home/christophe/.dropbox-dist/dropbox-lnx.x86_64-38.3.23/linuxffi.sys.compiled._linuxffi_sys.so'
dropbox: load fq extension '/home/christophe/.dropbox-dist/dropbox-lnx.x86_64-38.3.23/PyQt5.QtWebKit.so'
dropbox: load fq extension '/home/christophe/.dropbox-dist/dropbox-lnx.x86_64-38.3.23/PyQt5.QtCore.so'
dropbox: load fq extension '/home/christophe/.dropbox-dist/dropbox-lnx.x86_64-38.3.23/PyQt5.QtNetwork.so'
dropbox: load fq extension '/home/christophe/.dropbox-dist/dropbox-lnx.x86_64-38.3.23/PyQt5.QtGui.so'
dropbox: load fq extension '/home/christophe/.dropbox-dist/dropbox-lnx.x86_64-38.3.23/PyQt5.QtWebKitWidgets.so'
dropbox: load fq extension '/home/christophe/.dropbox-dist/dropbox-lnx.x86_64-38.3.23/PyQt5.QtWidgets.so'
dropbox: load fq extension '/home/christophe/.dropbox-dist/dropbox-lnx.x86_64-38.3.23/PyQt5.QtPrintSupport.so'
dropbox: load fq extension '/home/christophe/.dropbox-dist/dropbox-lnx.x86_64-38.3.23/PyQt5.QtDBus.so'
Done!
The icon is now the fancy one, but context menu has no text and is not clickable.
Both mentioned packages are installed.