I was meaning to point you to these parts of the first question in the Frequently Asked Questions:
I found a bug. What do I do?
Please turn on debugging mode (in the UpdraftPlus options page) and then try again, and after that send me the log if you can find it (there are links to download logs on the UpdraftPlus settings page; or you may be emailed it; failing that, it is in the directory wp-content/updraft, so FTP in and look for it there). If you cannot find the log, then I may not be able to help so much, but you can try - include as much information as you can when reporting (PHP version, your blog's site, the error you saw and how you got to the page that caused it, etcetera).
If you know where to find your PHP error logs (often a file called error_log, possibly in your wp-admin directory (check via FTP)), then that's even better (don't send me multi-megabytes; just send the few lines that appear when you run a backup, if any).