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I have installed caja-dropbox package on my Ubuntu MATE 19.04 system as simple as sudo apt-get install caja-dropbox but it does not start from GUI and continuously asks me about Caja restart.

When starting from the terminal it shows the following error messages:

$ caja-dropbox start -i
Starting Dropbox...dropbox: locating interpreter
dropbox: logging to /tmp/dropbox-antifreeze-lmlIjG
dropbox: initializing
dropbox: initializing python 3.7.2
dropbox: setting program path '/var/lib/dropbox/.dropbox-dist/dropbox-lnx.x86_64-81.4.195/dropbox'
dropbox: setting home path '/var/lib/dropbox/.dropbox-dist/dropbox-lnx.x86_64-81.4.195'
dropbox: setting python path '/var/lib/dropbox/.dropbox-dist/dropbox-lnx.x86_64-81.4.195:/var/lib/dropbox/.dropbox-dist/dropbox-lnx.x86_64-81.4.195/python-packages-37.zip'
dropbox: python initialized
dropbox: running dropbox
dropbox: setting args
dropbox: applying overrides
dropbox: running main script
dropbox: load fq extension '/var/lib/dropbox/.dropbox-dist/dropbox-lnx.x86_64-81.4.195/cryptography.hazmat.bindings._constant_time.cpython-37m-x86_64-linux-gnu.so'
dropbox: load fq extension '/var/lib/dropbox/.dropbox-dist/dropbox-lnx.x86_64-81.4.195/cryptography.hazmat.bindings._openssl.cpython-37m-x86_64-linux-gnu.so'
dropbox: load fq extension '/var/lib/dropbox/.dropbox-dist/dropbox-lnx.x86_64-81.4.195/cryptography.hazmat.bindings._padding.cpython-37m-x86_64-linux-gnu.so'
dropbox: load fq extension '/var/lib/dropbox/.dropbox-dist/dropbox-lnx.x86_64-81.4.195/psutil._psutil_linux.cpython-37m-x86_64-linux-gnu.so'
dropbox: load fq extension '/var/lib/dropbox/.dropbox-dist/dropbox-lnx.x86_64-81.4.195/psutil._psutil_posix.cpython-37m-x86_64-linux-gnu.so'
dropbox: load fq extension '/var/lib/dropbox/.dropbox-dist/dropbox-lnx.x86_64-81.4.195/linuxffi.pthread._linuxffi_pthread.cpython-37m-x86_64-linux-gnu.so'
dropbox: load fq extension '/var/lib/dropbox/.dropbox-dist/dropbox-lnx.x86_64-81.4.195/cpuid.compiled._cpuid.cpython-37m-x86_64-linux-gnu.so'
dropbox: load fq extension '/var/lib/dropbox/.dropbox-dist/dropbox-lnx.x86_64-81.4.195/tornado.speedups.cpython-37m-x86_64-linux-gnu.so'
dropbox: load fq extension '/var/lib/dropbox/.dropbox-dist/dropbox-lnx.x86_64-81.4.195/apex._apex.cpython-37m-x86_64-linux-gnu.so'
Traceback (most recent call last):
 File "dropbox/client/main.pyc", line 18, in <module>
 File "dropbox/client/features/catalina_migration/catalina_migration_controller.pyc", line 19, in <module>
 File "dropbox/client/features/catalina_migration/catalina_account_context.pyc", line 13, in <module>
 File "dropbox/client/features/catalina_migration/alert_dialog.pyc", line 10, in <module>
 File "dropbox/client/features/file_locking/base_file_locking_alert.pyc", line 14, in <module>
 File "dropbox/client/features/legacy_ui_launcher.pyc", line 21, in <module>
 File "dropbox/client/configuration/manager.pyc", line 45, in <module>
 File "dropbox/client/configuration/utils.pyc", line 27, in <module>
 File "dropbox/client/notifications/loud_notifications/do_not_disturb/model.pyc", line 51, in <module>
 File "ui/common/preferences.pyc", line 21, in <module>
 File "dropbox/client/authentication/manager.pyc", line 36, in <module>
 File "dropbox/client/authentication/control_flow_helpers.pyc", line 3, in <module>
 File "dropbox/client/high_trace.pyc", line 53, in <module>
 File "dropbox/client/logging/dbxlog_logger.pyc", line 11, in <module>
 File "apex/dbxlog.pyc", line 14, in <module>
 File "<_bootstrap_overrides>", line 153, in load_module
ImportError: libdropbox_apex.so: cannot open shared object file: No such file or directory
!! dropbox: fatal python exception:
['Traceback (most recent call last):n', ' File "dropbox/client/main.pyc", line 18, in <module>n', ' File "dropbox/client/features/catalina_migration/catalina_migration_controller.pyc", line 19, in <module>n', ' File "dropbox/client/features/catalina_migration/catalina_account_context.pyc", line 13, in <module>n', ' File "dropbox/client/features/catalina_migration/alert_dialog.pyc", line 10, in <module>n', ' File "dropbox/client/features/file_locking/base_file_locking_alert.pyc", line 14, in <module>n', ' File "dropbox/client/features/legacy_ui_launcher.pyc", line 21, in <module>n', ' File "dropbox/client/configuration/manager.pyc", line 45, in <module>n', ' File "dropbox/client/configuration/utils.pyc", line 27, in <module>n', ' File "dropbox/client/notifications/loud_notifications/do_not_disturb/model.pyc", line 51, in <module>n', ' File "ui/common/preferences.pyc", line 21, in <module>n', ' File "dropbox/client/authentication/manager.pyc", line 36, in <module>n', ' File "dropbox/client/authentication/control_flow_helpers.pyc", line 3, in <module>n', ' File "dropbox/client/high_trace.pyc", line 53, in <module>n', ' File "dropbox/client/logging/dbxlog_logger.pyc", line 11, in <module>n', ' File "apex/dbxlog.pyc", line 14, in <module>n', ' File "<_bootstrap_overrides>", line 153, in load_modulen', 'ImportError: libdropbox_apex.so: cannot open shared object file: No such file or directoryn'] (error 3)

What should I do to get Dropbox working again?

The problematic library libdropbox_apex.so is located in the /var/lib/dropbox/.dropbox-dist/dropbox-lnx.x86_64-81.4.195 folder.

But there is one strange thing here - it do not have executable permissions. This is how it differs from other libraries:

$ ls -al libdropbox_apex.so
-rw------- 1 root root 7138143 сен 18 10:24 libdropbox_apex.so

To fix the problem we need to set read and execution permissions for this file:

sudo chmod a+rx /var/lib/dropbox/.dropbox-dist/dropbox-lnx.x86_64*/libdropbox_apex.so

Update 20191109: the number of problematic libraries - increased to 2, now with Dropbox 84.4.170 we need to change permission of libdropbox_sqlite_ext.so too:

sudo chmod a+rx /var/lib/dropbox/.dropbox-dist/dropbox-lnx.x86_64*/libdropbox_sqlite_ext.so

and then start it again from GUI or with caja-dropbox start -i.

Notes:

1) this method is also applicable for 16.04 LTS, 18.04 LTS and
19.10.

2) I reported this to LaunchPad as bug 1845876.

3) Ticket on dropbox support site was created on 20191005 but is still not resolved.