Hello, I'm also having what I believe is the same exact problem and are getting the same errors. It all started after I upgraded to 6.3.1. I have started another topic yesterday but got no responses so if you don't mind I will also post my diagnostics in the event our issues are related and additional information can shed light on the cause.
I guess in the interim, I can disable the use of cache for my media directories and read up more on the threads you already found.
Thanks!
noahsarc-diagnostics-20170215-0829.zip