R 3.5 is not supported by Exploratory yet, so if you happen to have installed R 3.5 separately then you will not be able to open the projects.
We are going to support it officially with the next release Exploratory 5.0, which is due in June.
If you can’t wait till then, then we have an internal development build available. Shoot us an email at support@exploratory.io, we’ll send you the download.
Cheers,
-Kan