" do you really recommed to me that i use a local (hard) drive for the boxcryptor folder ? And then i synchronize the local drive with an additional program with the cloud ? "
Yes, because rsync-based file transfer configurations are more efficient than internet-smb/cifs-drive-based transfer configurations, the solution sketched in a previous post is more efficient in most aspects I can think off, it also gives you some extra redundancy (more local copies), which is often a necessity, I think.
You also wrote:
" If i understood this right, then you lead the sales argument from boxcryptor, which is written on the homepage, ad absurdum. "
About your boxcryptor quote: "Optimized for Cloud Storage":
Note that systems like Google "Drive", mentioned in that quote, are not real mounted drives / volumes: such systems consist of a client-side software program that ... synchronizes one or more local directories to their storage infrastructure. Most efficient and stable storage systems work like that. The underlying synchronization mechanism is often rsync-like.
Using rsync based synchronization implicitly ( login + interval + logout settings ) or occasionally explicitly ( "sync now" ), we hardly have to pay attention to the syncs, it just works.
There is more to say in favor of the rsync approach, but here is a final remark regarding the Thunderbird email client you mentioned:
In case users prefer to use smtp / pop3, an email client like Thunderbird uses the so called "mbox" format. This mbox format is one of the possible email standards. It's characterized by the fact that all emails in a Thunderbird folder are stored in a single ( mbox ) file. Including the attachments of emails, PDFs, photos, videos, etc. Therefore, an email folder will often be 10's, 100's or even more MB's. You can do the math what happens each time you open an email that's part of a (remote) Thunderbird email folder, when using a drive-based solution.
Again, I would say use a rsync based solution, configuration is very easy, tools are free, browsing and transfer are ( much ) more efficient, you even may start to like it.