Thank you for your answers. mer's answer led me to the fact, that i tried only the gdb-interfaces for lldb3x. Using the lldb38 binary solved this issue for TrueOS, thank you.
But I'm still wondering, why this only works on TrueOS. I do have another machine running FreeBSD CURRENT, which behaves for the gdb thing the same way TrueOS does (see start post of this topic). Trying the exact same thing which worked for TrueOS resulted on the FreeBSD machine in not recognizing the breakpoints at all! If i use the debugger (even the gdb) on the console, all works fine. Curious...
For the records: it seems that qtcreator has some problems with symlinks. On my FreeBSD machine, the path to the qt-project contains a symlink, on TrueOS not. Providing a path without symlink in it, debugging works also on FreeBSD.