Restart the Battle.net desktop app to clear temporary issues that may cause this error.
How to resolve issues with slow patch downloads.
Troubleshooting steps for issues installing the PTR client
A permissions issue is preventing Battle.net desktop app from renaming a file during an update.
Troubleshooting your security programs can resolve issues preventing you from installing or using Blizzard software.
What to do if you are seeing an error after starting the game
Information regarding your computer shutting down or restarting while playing games
Out-of-date drivers and operating systems can cause a variety of issues, and are one of the first things to check when a game is having problems.
A guide on running a traceroute on Windows and Mac.