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．