If they do not have access, invite them to the app
If they do have access, remove them from the app and add them back – this will re-sync the access rules
If the above does not work, make sure the WebDAV protocol is not blocked within your network. This protocol is required by Studio Pro for version control to work, but it might be blocked by your proxy server or other software like a firewall.
If I dont use splintr and i have firewall i it possible to open via Mendix app ?