Firefox Download Issues – How to Solve File Saving Problems

This is not a full duplicate of this question or this question, though the base question is the same. However, those questions have no answers.

For over a half a year I have problems downloading files with Firefox. Generally when I click on a download link, a download dialog is opened asking me what to do, save it or open it. Immediately, there is also an error/warning pop-up (twice, but I only can see one at first) with only an OK button. Which looks like this with the message :

"C:\Users\%USER%\AppData\Local\Temp\wBBDv9K8.zip.part could not be saved because the source file could not be read.

Try again later or contact the server administrator."

Replace '%USER%' with the name of the user trying to download.

After closing the pop-up, the download dialog disappears and I have to minimize Firefox and open it again, to see the other pop-up.

Downloading the exact same file with Internet Explorer does work, but is not desired.

I tried all solutions from here.
I tried all solutions from here.
I tried disabling extensions.
I tried fixing permissions on the temp folder.
I tried making all files and folder in the profile directory non-readonly.
I tried running firefox as administrator.
I tried disabling my firewall.
I tried "Save link as…".
I tried selecting a different downloads folder. (though it is clear that is not the problem)
I tried resetting firefox. (via about:support).
I tried clearing the setting "network.http.accept-encoding" in about:config.

The resetting seemed to work at first until I tried to update flash, which was the first occurence of the problem since the reset and after a few other downloads. The problem with updating flash is that you cannot save the file, because flash tries that himself.

Very rarely though, the downloading of a file does work and I do not receive the error message. However, this chance feels like 1 in 100.

On the moment of writing this question I am using firefox 44.0, but this problem is already a few versions old at least.
Also, this problem did not occur before when using the same extensions.
In the meanwhile I upgraded from Windows 7 to Windows 10, but the problem still persists.

I am also able to ignore the warning and click on the download dialog and then click OK. This puts the download in some corrupt state which I cannot cancel. The only way to remove it is closing Firefox.

Current workaround:
After a download failed, I can go to the download library by clicking on the blue download icon. I can then click on the retry icon and the download succesfully finished. This only works however, when I click on the OK button of the dialog and therewith dismissing the download dialog.

I'm running out of options, what more can I try?
Is it also possible to disable 'predownloading' by skipping the temp folder and directly download to my downloads folder, starting when I click on OK button on the download dialog?