Cannot attach files in Outlook Web App (OWA)

Hi,

I'm having trouble attaching files to the Outlook Web App (OWA). When I try to attach a file, it gets forever, and then nothing, just an error:

"The action you tried to perform couldn't be completed because there's a configuration problem on the server. If the problem continues, contact your helpdesk"

I notice that I can attach small files (a few KiloBytes) but if the file is bigger than 50 KB, I get the above error.

I also tried with the Internet Explorer Developer Tools to monitor the traffic, and I found the below log:

From the URL above, I think is a proxy issue of the Exchange Web Services (EwsProxy), but when I go to Internet Options > Connections > Lan Settings, the Proxy server is disabled (unchecked)...

I've been days trying to figure out this problem, but I've been unable to find a solution.

Any help or thoughts are appreciated,

Thanks