to see if the version found by default is the one from RStudio or not ?
And can you provide the full pandoc command ?
I suspect there is something related to network drive. This does not mean that pandoc is not found but that there is a path seen as \\ and that it can't be found.
pandoc.exe: \\: openBinaryFile: does not exist (No such file or directory)
This could happen with one of the ressource in a network drive or something like that. This was / is a pandoc issue, I am not sure where we are on it.