RStudio does not like Uniform Naming Convention (UNC) paths. This can be a problem if, for example, you install it under Citrix. The solution is to create a suitable environment file. This is what worked for me: I created an .Renviron file in my Documents folder on the Citrix remote drive. The file had the following contents:
Simple. After that, the flurry of error and warning messages at startup disappeared and I was able install packages without any problems.