It's a little frustrating to see that no one ever bothered to come back and answer this. I'm now having this issue myself, and was hoping to see some king of answer, and instead see a link that doesn't do much.

My host recently migrated to a new server, and I started getting this progress bar error after that. According to the documentation in the link you provided, it is because the new server is write-caching. It gave two suggested fixes:

1. Add the following to the htaccess:

Code:
<IfModule mod_security.c>  
  # Turn off mod_security filtering.  
  SecFilterEngine Off  
 
  # The below probably isn't needed,  
  # but better safe than sorry.  
  SecFilterScanPOST Off  
</IfModule>
That code was already in the htaccess, so that isn't the solution. The second suggestion was to add the following to htaccess:

Code:
<IfModule mod_security.c>  
  SetEnvIfNoCase Content-Type \  
  "^multipart/form-data;" "MODSEC_NOPOSTBUFFERING=Do not buffer file uploads"  
</IfModule>  

<IfModule mod_gzip.c>  
  mod_gzip_on No  
</IfModule>
I added that and did a test upload of a file that was just a few megabytes, and it seemed ok. But what I've learned is that any file of any size at all, even just 10mb, will not go through. It will spend like 20 minutes trying to convert, then it will take me to the signup page. If I try to log in from that page, I get some kind of uber uploader error telling me that I have an invalid session id. It really makes no sense at all.

What do I need to do to fix this problem? Please just give me a direct answer and not point me to a link that goes to a page containing a link to another page. I've already been there and it didnt get any results.