Press F7 and enter "a/b". Expected: the directories ./a and ./a/b are created. Unexpected result: the directories ./a and ./b are created and there is a message "Error: The folder ./b does not exist".
What is your KIO version? Can't reproduce on KF 5.46. I also used spaces in folder names. Also, can you update to Krusader 2.7.0?
Fixed in v2.7. See https://phabricator.kde.org/D7610.