I'm currently using 2018.3 but this was replicated in 2019.1 and 2018.1

I have recently opened a file I created a couple of weeks ago to find that Tableau Prep tells me that I don't have permission to write to the file

The source data, tfl file and output data are all located on mapped network drives accessed via VPN. I have full permissions for the directories in use. I have checked with IT and there have been no changes to security settings in the last couple of weeks since I first created the file.

I can confirm that the first time I create an output (both csv and hyper), the file is written successfully, but after a moment it then comes up with the error message indicator, and if I try to run the flow again I get the error.

More info - when I save the same file to a local drive, I get the popup asking whether I would like to overwrite the file, however on our work server there is no popup instead there is just the above error message.

I can't find this listed as a bug but am not sure whether there is something I'm doing wrong.