@mihha u're totally right
i have first started a conversation here, than figured out that i should contact AcyMailing straight - so I have accidentally duplicated channel...
Anyway, we have discovered that problem was (probably) in a special sign used in Article's Title (see attached) - it made that neither the "Send a test" button nor the "Save and continue" button, neither "Summary" was working.
Once we have left out that Article from Newsletter, it worked...
Problem solved thanks to You and Thomas!
U were very fast with res responses - I appreciate that!