File uploading works in editor, but not in build

Hello,

I'm trying to implement a function in my project that would save a screenshot and then send it to a pre-defined location with the WWW class. Sounds relatively simple, but it's giving me a hard time. The problem is - when I start my app in the editor, it works fine, uploads the file and everything. When I build it, however, it takes the screenshot, but doesn't upload it anymore. The WWW class throws an error "Open file error: Couldn't open file", which is vague at best. Anybody got an idea what I'm doing wrong here? Here's the code I'm using for my Screenshot Manager:

     public IEnumerator TakeScreenShot(string name)
     {
         Debug.Log("Starting TakeScreenShot");
         string fileName = name + " " + System.DateTime.Now.ToString().Replace(":", ".").Replace("/", "-") + ".png";
         
         Debug.Log("Filename set - " + fileName);
         
         Debug.Log("Coroutine 1 starting...");
         yield return StartCoroutine("HideCam");
         Debug.Log("Coroutine 1 ended.");
         
         Debug.Log("Coroutine 2 starting...");
         yield return StartCoroutine("CapScreen", fileName);
         Debug.Log("Coroutine 2 ended.");
         
         Debug.Log("Coroutine 3 starting...");
         yield return StartCoroutine("ShowCam");
         Debug.Log("Coroutine 3 ended.");
 
         
         System.IO.FileInfo fi = new System.IO.FileInfo(fileName);
         
         GameObject fm = GameObject.Find("FileUpload");
         
         FileUpload uploadManager = (FileUpload)fm.GetComponent("FileUpload");
         
         string existingFile;
         
         existingFile = fi.FullName.Replace("\\","/");
         
         Debug.Log("File name generated - " + existingFile);
         Debug.Log("Waiting for 0.5 seconds...");
         yield return new WaitForSeconds(0.5f);
         Debug.Log("Wait edned. Starting upload...");
         uploadManager.UploadFile(existingFile, uploadLocation);
         
         
         yield return new WaitForSeconds(0.5f);
         
         if(uploadManager.GetSuccess() == true)
                 fi.Delete();
         
         yield return 1;
         
         uploadManager.ResetSuccess();
 
     }

For uploading I use WWW + php script combination.