Free Encrypted Cloud Storage for Windows, Linux and Mac

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In order to share and store sensitive data of my startup company I was searching for a cloud storage solution which offers at least 5 GB of storage for free, the data should be client-sided encrypted (which means that there is no chance for the cloud provider to access your data) and as we are using Windows 10, Ubuntu 16.04 and Mac OS in the company there must be a good client program for all of them. (Directly to: How to setup)