At the moment data is not encrypted on a Solid server, so is accessible to anyone who has access to the server (such as a provider, server admin etc). So if you want to store sensitive data you will need to deal with those issues, as well as all the other security issues related to servers exposed to the internet.
There may be plans to improve this area, but none are public AFAIK.
My own approach to solving this and other issues with pods as a service is to put Solid on SAFE Network, which is fully encrypted and does not allow access to data to anyone without explicit permission (no server, all data at rest is encrypted, all data in transit is encrypted), and avoids the problems with servers by eliminating them in favour of a secure decentralised network.
The best way to contact Melvin is to mention him on the Solid gitter chat, but he may just be busy with other things atm.