Can you connect with any other SFTP or FTP client?
First of all: Happy 2018! And thanks for replying.
Followed your suggestion: with other tools: similar problems.
Possibly my OpenSSH-server-installation was not configured well or something else. Re-installed it and now everything works fine.
My apologies for asking before I tried everything else: I'm relatively new to all this stuff: have end-user experience with linux, but now since some weeks experimenting with setting up web-servers on linux, configuring firewalls etc.
I have to login to give my thanks to you. You help me out a lot when I get lost. Thanks so much, bro.