Don't know exactly how, but it turns out this problem began after I tightened-up some security features within my PC and my anti-virus program.

Full-functionality of RP is back after relaxing a few settings back, but don't know which particular setting(s) caused this.

So, I've now learned something new about paying more attention to
inter-relationships among different programs possibly affecting others when settings are changed. And now after making a change to one program's settings, I start each/all programs to check if changed.