Based on Jeff's blog here https://www.thatjeffsmith.com/archive/2012/08/sql-developer-trick-renaming-your-worksheets/ and other topics I tried it too - and got his result:
What went wrong?
EDIT:
I just saw that it is case-sensitive. If only one character is upper case (e.g. formatted with Ctrl-F7 or "set worksheetnamE hello") the result is displayed above.
Is it a bug or a feature?
Thanks for advice!