SE named two known adware programs that can cause the problem - support said this:

"If you encounter the error messages [30605], [20448], [10009] or [10019] when you try to update the game client, your computer's security may have been compromised by adware.

We are aware of two such programs; "Savings Bull" and "Quick Knowledge" which have been known to cause these issues but other malicious programs may also be causing your issue.
To solve this issue, please uninstall these programs, should you find them on your system, and be sure to regularly scan your computer for any unwanted programs or infections."

The one on mine was called SupraSaver.

Take a look through your control panel and see if there's any program with those names, or any program that's suspicious. Run a scan with your antivirus, and if it can't pick anything up, download AdwCleaner, have it scan and then delete any adware it detects. It will reboot your computer to complete the removal. After your computer has restarted, open task manager (ctrl + alt + del) and look through the processes for the name of anything above (for example, it was suprasaver.exe for me) or any other suspicious program you found. If you find the name, stop the process. Then try starting up your game again. That's what worked for me, although I ran Adwcleaner and rebooted a few times.