I Cannot Play Any Videos On My Mac Computer Using Multiple Browsers

Until a few weeks ago I was able to play videos on Facebook, YouTube and many other websites on my Mac Mini Server (mid-2011) running High Sierra. Now when I attempt to play videos they either just sit there or show a progress bar. They never play. This happens on all the browsers I have installed: Safari, Chrome, Firefox and Opera. I have the latest versions of all of them installed. I have cleaned the cache of all of them. I have turned off hardware acceleration as suggested on some blogs. I have the latest versions of Flash (NPAPI, PPAPI) installed. I don't have this problem on my MacBook Air (mid 2011). The only web blocking I have turned on using Sophus is for websites known to have malware. I run no antivirus software. At this point I assume there is an OS setting somewhere on my Mac Mini Server that I need to change to get this working again. UPDATE December 5, 2018: This morning I started using Facebook, YouTube, etc. and had no problems playing videos. I checked on Opera and Safari, the two browsers I use regularly. I did nothing else other than what I stated in this question. I have no idea what happened other than it was unique to my Mac Mini Server.