You do not have to un-install the IDrive app for updating to latest version. You can just upgrade IDrive App to 2.03.19. Once app is updated you can then update your DSM to 6.0. It should work as expected.
Please logout from IDrive App. Go to Synology browser and make sure NAS user which you are using for IDrive Synology App, is having read/write permission to web folder. This will address the error that says failed to create directory.
@naifyboy We are continuously working on enhancing IDrive for Synology app so that we can provide better user experience. We are committed to resolve users issues in minimum possible time. We do have apps for both QNAP and Netgear NAS devices as well. However some users are facing issue with Synology app due to consistent development of Synology DSM. With the current release (Version 2.03.19) you should have better experience with backup/restore operations.
We request you to raise a support ticket from the following link. Our Tier 2 technical support team will schedule a remote session with our development team who can further investigate and provide you with a resolution: https://www.idrive.com/support