Warning: Your WordPress installation has a problem with outputting extra whitespace. This can corrupt backups that you download from here. Follow this link for more information
If UpdraftPlus has warned you about extra white-space, then this page explains the meaning of this warning. N.B. This problem does not affect the backing up process or the integrity of your backups. It only affects downloading your backups using the UpdraftPlus dashboard page, and can be worked around; please read on!
Some plugins or changes that you make to WordPress can cause extra spaces and/or line feeds (return characters) to be added to the output of every WordPress page. This is not usually a problem since they are invisible.
However, this is a problem when data other than text is being sent – e.g. when you are downloading a backup file from the UpdraftPlus console. Here, extra characters can change and corrupt the file. For example, if you add extra spaces into a zip archive, then it cannot be opened until they are removed again.
Firstly, if you are having this problem, then you may be able to directly download your backup files via FTP (i.e. instead of downloading them via the WordPress dashboard). This will circumvent the problem. After downloading from cloud storage to your web server within the UpdraftPlus page, then instead of downloading from there to your computer, instead just log into your FTP account, and look in wp-content/updraft (or, if you changed the default, wherever you configured to store your backups).
If you can, it is best to find and fix the problem. It is most likely to be in one of your plugins. You should de-activate them all (except UpdraftPlus, of course), and then re-load the UpdraftPlus page, and see if the problem has gone away. Assuming that it does, you can then re-activate the plugins one-by-one, and find out which one causes the error to return. You can then report the problem to the plugin author.
If the problem is in none of your plugins, then it must be in some other part of your WordPress install, that has been modified from the default. Hunting it down can be frustrating – but it’s in there somewhere… you could also temporarily switch theme to a default theme (e.g. TwentyTwelve, TwentyThirteen). If that doesn’t find it, then try checking your wp-config.php file.