MP4 Files not uploading

I have a version 2.1 install and recently I've been getting this error when trying to upload MP4 files to items:

Omeka_Storage_Exception

    Unable to store file.

#0 [internal function]: Omeka_Storage_Adapter_Filesystem->store('/tmp//fullsize_...', 'fullsize/89f85c...')
#1 /omeka/www/data/application/libraries/Omeka/Storage.php(67): call_user_func_array(Array, Array)
#2 /omeka/www/data/application/models/File.php(435): Omeka_Storage->__call('store', Array)
#3 /omeka/www/data/application/models/File.php(435): Omeka_Storage->store('/tmp//fullsize_...', 'fullsize/89f85c...')
#4 /omeka/www/data/application/models/Job/FileProcessUpload.php(23): File->storeFiles()
#5 /omeka/www/data/application/libraries/Omeka/Job/Dispatcher/Adapter/Synchronous.php(25): Job_FileProcessUpload->perform()
#6 /omeka/www/data/application/libraries/Omeka/Job/Dispatcher/Default.php(136): Omeka_Job_Dispatcher_Adapter_Synchronous->send('{"className":"J...', Array)
#7 /omeka/www/data/application/models/File.php(209): Omeka_Job_Dispatcher_Default->send('Job_FileProcess...', Array)
#8 [internal function]: File->afterSave(Array)
#9 /omeka/www/data/application/libraries/Omeka/Record/AbstractRecord.php(277): call_user_func(Array, Array)
#10 /omeka/www/data/application/libraries/Omeka/Record/AbstractRecord.php(550): Omeka_Record_AbstractRecord->runCallbacks('afterSave', Array)
#11 /omeka/www/data/application/models/Item.php(322): Omeka_Record_AbstractRecord->save()
#12 /omeka/www/data/application/models/Builder/Item.php(204): Item->saveFiles()
#13 /omeka/www/data/application/libraries/globals.php(554): Builder_Item->addFiles('Filesystem', Array, Array)
#14 /omeka/www/data/plugins/Dropbox/DropboxPlugin.php(100): insert_files_for_item(Object(Item), 'Filesystem', Array, Array)
#15 [internal function]: DropboxPlugin->hookAfterSaveItem(Array)
#16 /omeka/www/data/application/libraries/Omeka/Plugin/Broker.php(157): call_user_func(Array, Array)
#17 /omeka/www/data/application/libraries/Omeka/Record/AbstractRecord.php(298): Omeka_Plugin_Broker->callHook('after_save_item', Array)
#18 /omeka/www/data/application/libraries/Omeka/Record/AbstractRecord.php(550): Omeka_Record_AbstractRecord->runCallbacks('afterSave', Array)
#19 /omeka/www/data/application/libraries/Omeka/Controller/AbstractActionController.php(175): Omeka_Record_AbstractRecord->save(false)
#20 /omeka/www/data/application/controllers/ItemsController.php(88): Omeka_Controller_AbstractActionController->editAction()
#21 /omeka/www/data/application/libraries/Zend/Controller/Action.php(516): ItemsController->editAction()
#22 /omeka/www/data/application/libraries/Zend/Controller/Dispatcher/Standard.php(308): Zend_Controller_Action->dispatch('editAction')
#23 /omeka/www/data/application/libraries/Zend/Controller/Front.php(954): Zend_Controller_Dispatcher_Standard->dispatch(Object(Zend_Controller_Request_Http), Object(Zend_Controller_Response_Http))
#24 /omeka/www/data/application/libraries/Zend/Application/Bootstrap/Bootstrap.php(97): Zend_Controller_Front->dispatch()
#25 /omeka/www/data/application/libraries/Zend/Application.php(366): Zend_Application_Bootstrap_Bootstrap->run()
#26 /omeka/www/data/application/libraries/Omeka/Application.php(79): Zend_Application->run()
#27 /omeka/www/data/admin/index.php(28): Omeka_Application->run()
#28 {main}

I have successfully uploaded MP4 files in the past, and they are still fully functional, and I can still upload other file types just fine. This problem is happening on both our development server and our public server, and I had our server admin change the php max_file_size settings to accomodate these large files, so that isn't the issue.

I had recently been playing around in my theme with making some custom functions, but as a test I have completely uninstalled by custom theme and all associated php files I had made changes to, and the problem persists. I even deactivated and uninstalled most of my plugins, but still no luck. I also deleted and reinstalled the latest version of the Dropbox plugin. I've also tried uploading the MP4 files through CSV Import using a hyperlink. I think I'm running out of scenarios to test.

This problem just started recently, and like I said, I was able to upload MP4 files just fine before, and those files that are already in Omeka are working just fine. Any ideas?

Have you checked your PHP log (often the same as the Apache log)?

That "unable to store file" message generally occurs when some PHP-level function Omeka tries to use to move the uploaded files around fails. That failure should produce a Warning: or Error: message in PHP's log describing what happened.

You can upload other types of files fine? This kind of problem is most often caused by an issue with permissions, but that would affect all filetypes.