W3 Cache Plugin crashed website

After enabling page cache my website was a lot faster which was great.

But after enabling settings at the 1min 46sec mark of this video my website crashed: https://www.youtube.com/watch?v=l57BOt3BZjA

My website now reads

“The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator at root@localhost to inform them of the time this error occurred, and the actions you performed just before this error.

More information about this error may be available in the server error log.”