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Can't connect to DB anymore, Debian, Robo 3T, PuTTY

I can’t connect to MongoDB through PuTTY anymore. It worked before I started to practise backing up and restoring DB. The settings are the same in PuTTY nothing has changed.
Where should I start looking for the problem?
My setup: Windows notebook with Robo 3T, PuTTY (portforwarding activated), Debians 10 (with MongoDB), The IP of Debian is The IP of MongoDB
Status of db is running
My dbs overview

If no db is “selected” should Robo 3T be able to connect to this db?

Thank you.