Windows10多R版本环境下R 3.6.1无法访问CRAN仓库求助

之前在Windows 10系统上，R 3.6.1（64位）搭配CRAN和Rtools 3.5安装包一直正常，直到安装R 4.3.0和Rtools 4.3后，R 3.6.1再也无法从CRAN安装包。同一系统下R 4.3.0在RStudio中可正常安装新包，我已卸载R 3.6.1、删除库路径并重新安装，问题依旧，报错如下：

Warning: unable to access index for repository https://cran.rstudio.com/src/contrib:
cannot open URL 'https://cran.rstudio.com/src/contrib/PACKAGES'

已尝试以下解决方案，均无效：

• 禁用“HTTP安全下载方式”
• 更换CRAN仓库

测试操作及结果 #

1. 运行代码：

browseURL("https://cran.rstudio.com/src/contrib/PACKAGES")

浏览器可正常打开显示可用包列表的页面。

2. 运行代码：

getOption("repos")

响应结果：

CRAN 
"https://cran.rstudio.com/" 
attr(,"RStudio")
[1] TRUE

3. 运行代码：

install.packages("ggplot2", repos = "https://cloud.r-project.org")

响应结果：

Warning in install.packages :
  unable to access index for repository https://cran.rstudio.com/src/contrib:
  cannot open URL 'https://cran.rstudio.com/src/contrib/PACKAGES'
Installing package into ‘C:/Users/userName/Documents/R/win-library/3.6’
(as ‘lib’ is unspecified)
Warning in install.packages :
  unable to access index for repository https://cloud.r-project.org/src/contrib:
  cannot open URL 'https://cloud.r-project.org/src/contrib/PACKAGES'
Warning in install.packages :
  package ‘ggplot2’ is not available (for R version 3.6.1)
Warning in install.packages :
  unable to access index for repository https://cloud.r-project.org/bin/windows/contrib/3.6:
  cannot open URL 'https://cloud.r-project.org/bin/windows/contrib/3.6/PACKAGES'

重新安装R 3.6.1后的会话信息 #

R version 3.6.1 (2019-07-05)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 10 x64 (build 19045)

Matrix products: default

locale:
[1] LC_COLLATE=French_Canada.1252  LC_CTYPE=French_Canada.1252    LC_MONETARY=French_Canada.1252
[4] LC_NUMERIC=C                   LC_TIME=French_Canada.1252    

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

loaded via a namespace (and not attached):
[1] compiler_3.6.1 tools_3.6.1

内容的提问来源于stack exchange，提问作者lavaman