Suddenly my konsole became no-transparent and I went into
Settings > Manage Profiles > Edit Profile > Appearance, but I got the message that "This color scheme uses a transparent background which does not appear to be supported on your desktop".
I did not change anything before. So what I can do to solve this problem? Who can give me some advise?
Thank you all!

There seems no compositor active (afair konsole looks at that only at startup and some kwin versions "forgot" to claim that feature, not sure whether the latter made it into any release, though)