Insufficient permissions

Recently Chrome stopped downloading files with a "Failed Insufficient permissions" message.
I tried Firefox with the same result.
I tried Opera and it worked... for a while, then it produced the same message.
Same for Internet Explorer and Edge.
I have run out of browsers and I now cannot download anything.
I can save web pages but I cannot download pdf, epub, csv, exe.
I have tried fooling it by changing the extension to mhtml or jpg but I get the same message.
I don't know why this has started happening recently.
Can anybody please explain and provide solutions, either in terms of restoring permissions or switching off UAC?
Many thanks.

We're are you saving to as the permission on the download folder may be wrong or it may be temp folder some av block temp folder

I have tried several locations with the same result.
In any case, all these locations were fine a few days ago.