build error during archiver, filename mixed with path without slashes

[PerennialNovice]

I have a somewhat larger project (in sloeber) and am in the process of porting from AVR to STM32
Now I ran into a problem I think might be toolchain related:

build runs through fine, only after the archiver step it fails with a strange message

Finished building: C:\Sloeber\arduinoPlugin\packages\STMicroelectronics\hardware\stm32\2.7.1\cores\arduino\main.cpp
 
Finished building: C:\Sloeber\arduinoPlugin\packages\STMicroelectronics\hardware\stm32\2.7.1\cores\arduino\wiring_pulse.cpp
 
Starting archiver
"C:\Sloeber\arduinoPlugin\packages\STMicroelectronics\tools\xpack-arm-none-eabi-gcc\12.2.1-1.2/bin/arm-none-eabi-gcc-ar" rcs  "C:\sloeber_fs-Test\TestHead_STM32\Release\arduino.ar"  .\core\core\stm32\usb\hid\usbd_hid_composite.c.o .\core\core\stm32\usb\hid\usbd_hid_composite_if.c.o  .\core\core\stm32\usb\cdc\cdc_queue.c.o .\core\core\stm32\usb\cdc\usbd_cdc.c.o .\core\core\stm32\usb\cdc\usbd_cdc_if.c.o  .\core\core\stm32\usb\usb_device_core.c.o .\core\core\stm32\usb\usb_device_ctlreq.c.o .\core\core\stm32\usb\usb_device_ioreq.c.o .\core\core\stm32\usb\usbd_conf.c.o .\core\core\stm32\usb\usbd_desc.c.o .\core\core\stm32\usb\usbd_ep_conf.c.o .\core\core\stm32\usb\usbd_if.c.o  .\core\core\stm32\OpenAMP\virtio\virtio.c.o .\core\core\stm32\OpenAMP\virtio\virtqueue.c.o  .\core\core\stm32\OpenAMP\open-amp\rpmsg\rpmsg.c.o .\core\core\stm32\OpenAMP\open-amp\rpmsg\rpmsg_virtio.c.o  .\core\core\stm32\OpenAMP\open-amp\remoteproc\remoteproc_virtio.c.o  .\core\core\stm32\OpenAMP\libmetal\generic\cortexm\sys.c.o  .\core\core\stm32\OpenAMP\libmetal\generic\condition.c.o .\core\core\stm32\OpenAMP\libmetal\generic\generic_device.c.o .\core\core\stm32\OpenAMP\libmetal\generic\generic_init.c.o .\core\core\stm32\OpenAMP\libmetal\generic\generic_io.c.o .\core\core\stm32\OpenAMP\libmetal\generic\generic_shmem.c.o .\core\core\stm32\OpenAMP\libmetal\generic\time.c.o  .\core\core\stm32\OpenAMP\libmetal\device.c.o .\core\core\stm32\OpenAMP\libmetal\init.c.o .\core\core\stm32\OpenAMP\libmetal\io.c.o .\core\core\stm32\OpenAMP\libmetal\log.c.o .\core\core\stm32\OpenAMP\libmetal\shmem.c.o  .\core\core\stm32\OpenAMP\mbox_ipcc.c.o .\core\core\stm32\OpenAMP\openamp.c.o .\core\core\stm32\OpenAMP\rsc_table.c.o .\core\core\stm32\OpenAMP\virt_uart.c.o .\core\core\stm32\OpenAMP\virtio_buffer.c.o .\core\core\stm32\OpenAMP\virtio_log.c.o  .\core\core\stm32\startup_stm32yyxx.S.o  .\core\core\avr\dtostrf.c.o  .\core\core\HardwareSerial.cpp.o .\core\core\IPAddress.cpp.o .\core\core\Print.cpp.o .\core\core\RingBuffer.cpp.o .\core\core\Stream.cpp.o .\core\core\Tone.cpp.o .\core\core\USBSerial.cpp.o .\core\core\VirtIOSerial.cpp.o .\core\core\WInterrupts.cpp.o .\core\core\WMath.cpp.o .\core\core\WSerial.cpp.o .\core\core\WString.cpp.o .\core\core\abi.cpp.o .\core\core\board.c.o .\core\core\core_debug.c.o .\core\core\hooks.c.o .\core\core\itoa.c.o .\core\core\main.cpp.o .\core\core\pins_arduino.c.o .\core\core\wiring_analog.c.o .\core\core\wiring_digital.c.o .\core\core\wiring_pulse.cpp.o .\core\core\wiring_shift.c.o .\core\core\wiring_time.c.o 
c:\sloeber\arduinoplugin\packages\stmicroelectronics\tools\xpack-arm-none-eabi-gcc\12.2.1-1.2\bin\../lib/gcc/arm-none-eabi/12.2.1/../../../../arm-none-eabi/bin/ar.exe: .corecorestm32usbhidusbd_hid_composite.c.o: No such file or directory
make[1]: *** [makefile:156: C:\sloeber_fs-Test\TestHead_STM32\Release\arduino.ar] Error 1
make: *** [makefile:141: all] Error 2
"C:/Sloeber/arduinoPlugin/tools/make/make -j12 all" terminated with exit code 2. Build might be incomplete

it seems it is looking for a file named .corecorestm32usbhidusbd_hid_composite.c.o which resembles a lot the first file with path .\core\core\stm32\usb\hid\usbd_hid_composite.c.o

maybe this is a slash/backslash - problem?