Dropbox Plugin Failure

Attempting to add a file to an item being edited results in error "Omeka_Storage_Exception" "Unable to Store File".
The following is the transcript of the 'stack trace'.
We previously used the Dropbox plugin with no problems with earlier versions of Omeka. Any clue for a resolution would be appreciated.
exception 'Omeka_Storage_Exception' with message 'Unable to store file.' in /var/www/html/omeka-2.4/application/libraries/Omeka/Storage/Adapter/Filesystem.php:119
Stack trace:
#0 [internal function]: Omeka_Storage_Adapter_Filesystem->store('/tmp/9cba5cb2d2...', 'original/9cba5c...')
#1 /var/www/html/omeka-2.4/application/libraries/Omeka/Storage.php(67): call_user_func_array(Array, Array)
#2 /var/www/html/omeka-2.4/application/models/File.php(424): Omeka_Storage->__call('store', Array)
#3 /var/www/html/omeka-2.4/application/models/File.php(424): Omeka_Storage->store('/tmp/9cba5cb2d2...', 'original/9cba5c...')
#4 /var/www/html/omeka-2.4/application/models/Job/FileProcessUpload.php(23): File->storeFiles()
#5 /var/www/html/omeka-2.4/application/libraries/Omeka/Job/Dispatcher/Adapter/Synchronous.php(25): Job_FileProcessUpload->perform()
#6 /var/www/html/omeka-2.4/application/libraries/Omeka/Job/Dispatcher/Default.php(136): Omeka_Job_Dispatcher_Adapter_Synchronous->send('{"className":"J...', Array)
#7 /var/www/html/omeka-2.4/application/models/File.php(203): Omeka_Job_Dispatcher_Default->send('Job_FileProcess...', Array)
#8 [internal function]: File->afterSave(Array)
#9 /var/www/html/omeka-2.4/application/libraries/Omeka/Record/AbstractRecord.php(277): call_user_func(Array, Array)
#10 /var/www/html/omeka-2.4/application/libraries/Omeka/Record/AbstractRecord.php(550): Omeka_Record_AbstractRecord->runCallbacks('afterSave', Array)
#11 /var/www/html/omeka-2.4/application/models/Item.php(335): Omeka_Record_AbstractRecord->save()
#12 /var/www/html/omeka-2.4/application/models/Builder/Item.php(204): Item->saveFiles()
#13 /var/www/html/omeka-2.4/application/libraries/globals.php(567): Builder_Item->addFiles('Filesystem', Array, Array)
#14 /var/www/html/omeka-2.4/plugins/Dropbox/DropboxPlugin.php(100): insert_files_for_item(Object(Item), 'Filesystem', Array, Array)
#15 [internal function]: DropboxPlugin->hookAfterSaveItem(Array)
#16 /var/www/html/omeka-2.4/application/libraries/Omeka/Plugin/Broker.php(157): call_user_func(Array, Array)
#17 /var/www/html/omeka-2.4/application/libraries/Omeka/Record/AbstractRecord.php(298): Omeka_Plugin_Broker->callHook('after_save_item', Array)
#18 /var/www/html/omeka-2.4/application/libraries/Omeka/Record/AbstractRecord.php(550): Omeka_Record_AbstractRecord->runCallbacks('afterSave', Array)
#19 /var/www/html/omeka-2.4/application/libraries/Omeka/Controller/AbstractActionController.php(229): Omeka_Record_AbstractRecord->save(false)
#20 /var/www/html/omeka-2.4/application/controllers/ItemsController.php(91): Omeka_Controller_AbstractActionController->editAction()
#21 /var/www/html/omeka-2.4/application/libraries/Zend/Controller/Action.php(516): ItemsController->editAction()
#22 /var/www/html/omeka-2.4/application/libraries/Zend/Controller/Dispatcher/Standard.php(308): Zend_Controller_Action->dispatch('editAction')
#23 /var/www/html/omeka-2.4/application/libraries/Zend/Controller/Front.php(954): Zend_Controller_Dispatcher_Standard->dispatch(Object(Zend_Controller_Request_Http), Object(Zend_Controller_Response_Http))
#24 /var/www/html/omeka-2.4/application/libraries/Zend/Application/Bootstrap/Bootstrap.php(105): Zend_Controller_Front->dispatch()
#25 /var/www/html/omeka-2.4/application/libraries/Zend/Application.php(384): Zend_Application_Bootstrap_Bootstrap->run()
#26 /var/www/html/omeka-2.4/application/libraries/Omeka/Application.php(79): Zend_Application->run()
#27 /var/www/html/omeka-2.4/admin/index.php(28): Omeka_Application->run()
#28 {main}

I have found another piece of information that may be helpful in debugging.
Omeka is clearly writing to the temporary directory (/tmp) leaving the processed image files, the original, thumbnail, square thumbnail and fullsize. It is also leaving a lot of small files pertaining to 'Zend'. Each file contains a line of text and numbers which may be diagnostic. A listing of the file names follows:
omeka_i18n_cache---internal-metadatas---Zend_LocaleC_en_US_date_gregorian_long
omeka_i18n_cache---internal-metadatas---Zend_LocaleC_en_US_date_gregorian_medium
omeka_i18n_cache---internal-metadatas---Zend_LocaleC_en_US_decimalnumber_
omeka_i18n_cache---internal-metadatas---Zend_LocaleC_en_US_month_gregorian_format_abbreviated_8
omeka_i18n_cache---internal-metadatas---Zend_LocaleC_en_US_month_gregorian_format_abbreviated_9
omeka_i18n_cache---internal-metadatas---Zend_LocaleC_en_US_month_gregorian_format_wide_3
omeka_i18n_cache---internal-metadatas---Zend_LocaleC_en_US_parentlocale_
omeka_i18n_cache---internal-metadatas---Zend_LocaleC_en_US_scientificnumber_
omeka_i18n_cache---internal-metadatas---Zend_LocaleL_en_US_symbols_
omeka_i18n_cache---Zend_LocaleC_en_US_date_gregorian_long
omeka_i18n_cache---Zend_LocaleC_en_US_date_gregorian_medium
omeka_i18n_cache---Zend_LocaleC_en_US_decimalnumber_
omeka_i18n_cache---Zend_LocaleC_en_US_month_gregorian_format_abbreviated_8
omeka_i18n_cache---Zend_LocaleC_en_US_month_gregorian_format_abbreviated_9
omeka_i18n_cache---Zend_LocaleC_en_US_month_gregorian_format_wide_3
omeka_i18n_cache---Zend_LocaleC_en_US_parentlocale_
omeka_i18n_cache---Zend_LocaleC_en_US_scientificnumber_
omeka_i18n_cache---Zend_LocaleL_en_US_symbols_

Are you able to upload files normally?

The error is basically saying Omeka was unable to move the file from the tmp directory to the Omeka "files/original" directory.

No, I cannot upload normally either. Omeka produces the same error and same symptoms (putting various files in /tmp directory).
I expect the processed files should be moved to ./omeka-2.4/files/fullsize, thumbnails &c. The permissions on each of these directories is 766, so there should be no problem writing to them. Is it possible Omeka is trying to move them somewhere else that is not writable?
Thanks.

The error message makes clear that the problem is happening when Omeka tries to copy the file from tmp to files/original. There's no other location involved.

766 doesn't really make sense to be the permissions for those directories, as the execute bit (the difference between 6 and 7) is necessary to enter a directory. In other words, even though 6 includes write privileges, without execute, you can't create files in the directory.

'chmod -R 777 files' worked.
It seems odd to me that the execute permission is necessary, but "it is what it is".
Thanks for your help.
Up and running!
For others: be sure to go into the omeka-2.4 directory or whatever you call it before invoking the chmod command. For example: cd /var/www/html/omeka-2.4