Debugger can't find source files

[Megatokio]

Hi,
i'm stuck with a problem debugging a project on macos10.12, currently using Qt5.9.1 and Qt Creator 4.3.1.
I maintain several Qt projects, all of them i can debug without problems. Only the largest project cannot be debugged.
The debugger does not stop on breakpoints and if the program crashes or is paused manually then the debugger only shows a disassembly of the .o file.
The reason seems to be that the debugger cannot find the sourcefile.

When starting debugging, i see this message in the Debugger Log Window (for one BP):

>(lldb) script theDumper.insertBreakpoint({"address":0,"command":"","condition":"","enabled":1,"expression":"","file":"/pub/Develop/Projects/zxsp-osx-test/OSX/Source/Application.cpp","function":"","id":0,"ignorecount":0,"line":424,"modelid":"1","oneshot":0,"token":6,"type":1})
>@
>result={token="6",lldbid="1",valid="1",hitcount="0",threadid="0",oneshot="0",condition="",enabled="1",valid="1",ignorecount="0",locations=[]}@
>6^
dNO LOCATIONS (YET) FOR BP Type: 0 [enabled] Number: 1 [pending] Hit: 0 times Type: 0 [enabled]

Up to ~Qt5.6 debugging this project worked.
I have already refactored the source tree in several ways, trying to match the layout of projects which can be debugged, without success.

Any help apprciated.