Dropbox が起動しなくなっていた．
exe を起動しても，10 秒程度で終了してしまう．
%Temp% にログがあったので見てみると，

bn.BUILD_KEY: Dropbox
bn.VERSION: 15.4.22
bn.DROPBOXEXT_VERSION: 3.0
bn.is_frozen: True
pid: 8680
cwd: u'C:\\Users\\Iwao\\AppData\\Roaming\\Dropbox\\bin'
     real_path=u'C:\\Users\\Iwao\\AppData\\Roaming\\Dropbox\\bin'
           	mode=040777	uid=0	gid=0
     parent	mode=040777	uid=0	gid=0
HOME: None
appdata: u'C:\\Users\\Iwao\\AppData\\Local\\Dropbox\\instance1'
         real_path=u'C:\\Users\\Iwao\\AppData\\Local\\Dropbox\\instance1'
               	mode=040777	uid=0	gid=0
         parent	mode=040777	uid=0	gid=0
dropbox_path: u'C:\\Users\\Public\\Documents\\Dropbox'
              real_path=u'C:\\Users\\Public\\Documents\\Dropbox'
                    	mode=040555	uid=0	gid=0
              parent	mode=040555	uid=0	gid=0
sys_executable: 'C:\\Users\\...\\Roaming\\Dropbox\\bin\\Dropbox.exe'
                real_path='C:\\Users\\...\\Roaming\\Dropbox\\bin\\Dropbox.exe'
                      	mode=0100777	uid=0	gid=0
                parent	mode=040777	uid=0	gid=0
trace.__file__: 'C:\\...\\Dropbox\\bin\\Dropbox.exe\\dropbox\\...\\boot_error.pyo'
                real_path='C:\\...\\Dropbox\\bin\\Dropbox.exe\\...\\boot_error.pyo'
                      	not found
                parent	not found
TMP: C:\Users\Iwao\AppData\Local\Temp
TEMP: C:\Users\Iwao\AppData\Local\Temp
tempdir: u'c:\\users\\iwao\\appdata\\local\\temp'
         real_path=u'c:\\users\\iwao\\appdata\\local\\temp'
               	mode=040777	uid=0	gid=0
         parent	mode=040777	uid=0	gid=0
Traceback (most recent call last):
  File "dropbox\client\main.pyo", line 5534, in main_startup
  File "dropbox\client\main.pyo", line 2264, in run
  File "ui\common\uikit.pyo", line 516, in create_ui_kit
  File "dropbox\client\ui\qt\__init__.pyo", line 13, in 
  File "PyQt5\QtCore.pyo", line 12, in 
  File "PyQt5\QtCore.pyo", line 10, in __load
ImportError: DLL load failed: 指定されたモジュールが見つかりません。

https://www.dropbox.com/install からダウンロードしてインストールすることにより，動作可能になった．