First of all, thanks for responding. The issue started around Fall Creator's Update time frame, that I am still unaware of how it happened. It's been almost a month since the update, and I haven't been able to run code in RStudio. To answer your questions:
It happens on all notebooks.
It fails in all three ways, menu, keyboard shortcut, and toolbar(in Run->Run selected lines), and Insert code chunk(R, SQL, Python, etc) in versions 1.1.383, 1.1.393preview. The latest 1.2.202 daily build fixed the insert button, but “run selected lines” still broken
It happens regardless of R environment, I tried both Microsoft R Open 3.4.2 and R 3.4.3.
If I have to guess why it happened, it is probably character encoding, such as line break encoding(or "", or ()) in non-english Win10 Fall Creator's Update, since I use a CN version of Win10 FCR x64 for obvious language compatibility reasons beyond my control. But I am not 100% sure about that since reverting back to pre Fall Creators would be a lot of work on my workstations, and RStudio worked before the FCR update in the Chinese x64 version of win10 creators update version. The chinese version of win10 has a keyboard input (left SHIFT button that constantly switches between EN, and CN input, and if it is shifted, the space, "", and () all look the same or weird, but the encodings are changed?, that's why Counter-Strike 1.6 always crashes in win10 CN version because of the SHIFT language input button altering encodings, that US developers never had to worry about that in their code)
I am not 100% sure about this speculation.
Since I am a software developer myself, are you guys at RStudio using the latest Visual Studio 2017 with appropriate windows 10 FCR patches for compiling RStudio? Since the release of 1.1.383 is released before win10 FCR (Nov 14), I wonder if you guys are not using the latest Windows libraries for compiling code, causing incompatibilities.(Like how Tensorflow 1.4.1 can't compile under win10 FCR right now) Also look into how the daily build 1.2.202 fixed the "Insert" button to insert R, SQL, Python code chunks. It isn't working in 1.1.383, or 1.1.393preview but now works in 1.2.202