I'm not sure where to post bug reports so I'm gonna put it here.

I start the download and download starts.
I stop the download part way through and continue it the next day.
The download URL changes because it is one of those file sharing websites.
I acquire the new URL and open the download properties and replace the old URL with the new URL.
I start the download and error message says 'Unknown server error' or along those lines I don't remember right now.
I delete the download entry and the incomplete file remains which is good.
I take the same new URL and I start a new download with it.
I did not specify to pre-allocate the file when I downloaded using the old URL which allows FDM to assume a resume point.
The download 'Ask' dialog gives me option to resume and I click resume.
It resumes without any problem. So in this case it CAN resume but it requires awkward user intervention. This is a minor issue and doesn't stop me from using FDM.