I found it. I did change /almost a year ago/ some lines (for a reason) in /etc/init.d/tc-config. Bad decision bad consequences. With the original tc-config it works without replacement of any part of busybox.
Sorry for having taken so much of your precious time. Thanks for the help curaga, bmarkus and tinypoodle!
[I am using since TC 4.0 the initram put togehter of four parts: base, kernel modules, apps, profile. I put the changed tc-config in the fourth part, that is why I overlooked it while tracking down the issue.]