This is the error I get on 2.4RC1. Note I also have problems on 2.3.7 that I can post here if you need.
Anybody can help?

[Warning] file_get_contents(/Users/xxx/Sites/silverstripe-v2.4.0-rc1/swfupload/javascript/swfupload_vars.js) [function.file-get-contents]: failed to open stream: No such file or directory
GET /silverstripe-v2.4.0-rc1/admin/assets/EditForm/field/Files/upload

Line 513 in /Users/xxx/Sites/silverstripe-v2.4.0-rc1/sapphire/core/Requirements.php
Source

504       else $this->customHeadTags[] = $html;
505    }
506    
507    /**
508     * Load the given javascript template with the page.
509     * @param file The template file to load.
510     * @param vars The array of variables to load. These variables are loaded via string search & replace.
511     */
512    function javascriptTemplate($file, $vars, $uniquenessID = null) {
513       $script = file_get_contents(Director::getAbsFile($file));
514       $search = array();
515       $replace = array();
516
517       if($vars) foreach($vars as $k => $v) {
518          $search[] = '$' . $k;
519          $replace[] = str_replace("\\'","'", Convert::raw2js($v));

Trace

* file_get_contents(/Users/xxx/Sites/silverstripe-v2.4.0-rc1/swfupload/javascript/swfupload_vars.js)
Line 513 of Requirements.php
* Requirements_Backend->javascriptTemplate(swfupload/javascript/swfupload_vars.js,Array,)
Line 129 of Requirements.php
* Requirements::javascriptTemplate(swfupload/javascript/swfupload_vars.js,Array)
Line 168 of SWFUploadConfig.php
* SWFUploadConfig::Bootstrap()
Line 20 of SWFUploadField.php
* SWFUploadField->__construct(UploadForm,Upload,,Array)
Line 265 of FileDataObjectManager.php
* FileDataObjectManager->getUploadFields()
Line 302 of FileDataObjectManager.php
* FileDataObjectManager->UploadForm()
Line 227 of FileDataObjectManager.php
* FileDataObjectManager->upload(SS_HTTPRequest)
Line 134 of RequestHandler.php
* RequestHandler->handleRequest(SS_HTTPRequest)
Line 152 of RequestHandler.php
* RequestHandler->handleRequest(SS_HTTPRequest)
Line 152 of RequestHandler.php
* RequestHandler->handleRequest(SS_HTTPRequest)
Line 147 of Controller.php
* Controller->handleRequest(SS_HTTPRequest)
Line 283 of Director.php
* Director::handleRequest(SS_HTTPRequest,Session)
Line 127 of Director.php
* Director::direct(/admin/assets/EditForm/field/Files/upload)
Line 127 of main.php

Are you referring to a file that I am attempting to upload?
I get the error BEFORE I can select any file, when I click on "upload file".

Duuuuh!!

Renamed "module-dataobject_manager-r394" to "dataobject_manager" and "module-swfupload-r375" to "swfupload" and it works.

Thank you!!!