I have had this problem since around 19/20 June. Downloads are announced as complete, but the files are small and, of course, corrupted.
If Opera cannot estimate the completion time, so it only shows a solid progress bar and the KB downloaded thus far, it will complete just fine. But when the progress can be measured and time estimated, it will fail.
The size and percentage vary, it seems. Usually, it's around 10-12MB most often for me.
I cannot try Opera on anything other than Macs, but it happens on all my computers, even with a clean new installation and mostly default settings.
I have sent a bug report but had no feedback.