Hi, I also have this problem of unable to complete an attachment download when the size is big (>2MB)
First off, I'm an end user, using the webmail service from my domain server supplier. So, I don't know how to get to the error log.
The problem happens only when I'm on a slow internet connection. If I switch to a faster connection, I can finish the download.
Also, when using slow connection, this problem doesn't occur on my other email client/provider, eg. Horde, Squirrelmail, Gmail, Yahoo Mail, Hotmail.
So, it only occurs on a combination of slow connection & Roundcube. Perhaps the download isn't persistent enough to continue on when met with some internet connection packet loss?