the issue is sorted out now. The SDK uses DEBUGGABLE_UDEBONLY to signal that the binary is debuggable but that flag is too restrictive and is applied only to binaries generated in the debug configuration. The SDK should use the DEBUGGABLE flag instead.
The only way to fix this is to edit the file C:\QtSDK\Symbian\SDKs\Symbian3Qt474\mkspecs\common\symbian\symbian-mmp.conf where at line 61 the text
should be replaced by
# Mark all debug executables debuggable.
MMP_RULES += DEBUGGABLE_UDEBONLY
# Mark all executables debuggable.
MMP_RULES += DEBUGGABLE