Joomla "copy file failed"

In Joomla! 3.x:

edit the libraries/src/Filesystem/Folder.php file

look for the following code, around lines 155-159:

if (!@copy($sfid, $dfid))
{
  throw new \RuntimeException('Copy file failed', -1);
}
and replace it with this one:

if (!@copy($sfid, $dfid))
{
  throw new \RuntimeException('Copy file failed  '.$sfid.' => '.$dfid, -1);
}

The code above will improve how the error message is displayed by adding the source and destination folders to it. This way, you will be able to know exactly which folders do not have enough permissions set.