SharePoint Web Part Error: "Unable to display this web part..."

Recently I came across web parts displaying nothing, no content, just the error: "Unable to display this Web Part. To troubleshoot the problem, open this Web page in a Microsoft SharePoint Foundation-compatible HTML editor such as Microsoft SharePoint Designer. If the problem persists, contact your Web server administrator."

After solving the problem, I decided to write this to save others a lot of lost time, hair pulling, and aggravation. I'm including my findings on several different reasons for this error as well as solutions for each. Hopefully one of these will help solve your problem as well.

For me it was a quick and easy solution (after doing hours of research): open the web part in SharePoint Designer and save it- that's all there was to it. Don't change anything, literally just open it up and save it.

So try that first and it may save you a huge amount of time. However, if that doesn't do the trick, the following is all the information, including causes and solutions, that I found. Hope it helps!

I came across a Wiki article that reported it was due to a July 2013 update found here: http://social.technet.microsoft.com/wiki/contents/articles/18589.sharepoint-2010-latest-windows-security-update-in-july-2013-breaks-the-sharepoint-xslt-system-web-part-pages.aspx#comments

Unfortunately, this is 2015, nearly 2 years later.

Upon further searching, I found this support article from MS: https://support.microsoft.com/en-us/kb/2639184 which seems to be very in depth for something that was probably just due to an update or server reset.

If you just restored a site, then this article may be helpful to you: http://blogs.msdn.com/b/spdsupport/archive/2008/06/06/how-to-fix-error-unable-to-display-this-web-part-on-data-views-after-restoring-site-from-backup.aspx