1. Log-in to public demo site.

Per the bug report template:

Additionally, do not rely on the demo instance for reproducing suspected bugs, as its data is prone to modification or deletion at any time.

Please revise your report above to include complete instructions that someone else can follow to reproduce the reported behavior. Otherwise, it will be closed as invalid.

Ok, It's a pity but the same bahaviour on all version and you even not need to do anything.

To reproduce you should do:

1. Create Panel1.1 Generic device
2. Add a new rear port splice named "Rare port splice to Panel1.2" with 12 positions
3. Add a new 12 front ports from 1 - 12 and attach it to newly created rare splice.
4. Add a new rear port splice "Rare port splice to Panel1.3" with 12 positions
5. Add a new 12 front ports from 24-35 and attach it to newly created rare splice.
6. Create rear port splice name "Rare port splice" Panel1.3 Generic device
7. Add a new rear port splice named "Rare port splice" with 12 positions
8. Add a new 12 front ports from 1 - 12 and attach it to newly created rare splice.
9. Create Panel1.3 Generic device
10. Add a new rear port splice named "Rare port splice" with 12 positions
11. Add a new 12 front ports from 1-12 and attach it to newly created rare splice.
12. Connect rear splice named "Rare port splice to Panel1.2" rear port splice name "Rare port splice" Panel1.2
13. Connect rear splice named "Rare port splice to Panel1.3" rear port splice name "Rare port splice" Panel1.3
14. Connect router port as was shown in image in first message (add port connection from NY-Router from port et-0/0/1 to Panel1.1-NY port 1 and port 24).

Thank you for making the requested updates to your report. I was able to reproduce the behavior on a v3.7.4 system using the new steps provided.

I also obtained the following traceback from v3.7.4, which may be helpful to the next person to work this issue:

Internal Server Error: /dcim/cables/add/
Traceback (most recent call last):
  File "/opt/netbox/venv/lib/python3.10/site-packages/django/core/handlers/exception.py", line 55, in inner
    response = get_response(request)
  File "/opt/netbox/venv/lib/python3.10/site-packages/django/core/handlers/base.py", line 197, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/opt/netbox/venv/lib/python3.10/site-packages/django/views/generic/base.py", line 104, in view
    return self.dispatch(request, *args, **kwargs)
  File "/opt/netbox/netbox/dcim/views.py", line 3194, in dispatch
    return super().dispatch(request, *args, **kwargs)
  File "/opt/netbox/netbox/netbox/views/generic/object_views.py", line 175, in dispatch
    return super().dispatch(request, *args, **kwargs)
  File "/opt/netbox/netbox/netbox/views/generic/base.py", line 26, in dispatch
    return super().dispatch(request, *args, **kwargs)
  File "/opt/netbox/netbox/utilities/views.py", line 106, in dispatch
    return super().dispatch(request, *args, **kwargs)
  File "/opt/netbox/venv/lib/python3.10/site-packages/django/views/generic/base.py", line 143, in dispatch
    return handler(request, *args, **kwargs)
  File "/opt/netbox/netbox/netbox/views/generic/object_views.py", line 269, in post
    obj = form.save()
  File "/opt/netbox/venv/lib/python3.10/site-packages/django/forms/models.py", line 542, in save
    self.instance.save()
  File "/opt/netbox/netbox/dcim/models/cables.py", line 240, in save
    trace_paths.send(Cable, instance=self, created=_created)
  File "/opt/netbox/venv/lib/python3.10/site-packages/django/dispatch/dispatcher.py", line 176, in send
    return [
  File "/opt/netbox/venv/lib/python3.10/site-packages/django/dispatch/dispatcher.py", line 177, in <listcomp>
    (receiver, receiver(signal=self, sender=sender, **named))
  File "/opt/netbox/netbox/dcim/signals.py", line 98, in update_connected_endpoints
    create_cablepath(nodes)
  File "/opt/netbox/netbox/dcim/utils.py", line 43, in create_cablepath
    cp = CablePath.from_origin(terminations)
  File "/opt/netbox/netbox/dcim/models/cables.py", line 547, in from_origin
    assert all(t.parent_object == terminations[0].parent_object for t in terminations[1:])
AssertionError

@ivanovuri have you had a chance to re-test this workflow in NetBox 4.0.x? We think there's a chance it got fixed as a side effect of other work, so before I mount a new effort to reproduce it, I thought I'd check with you.

This issue is reproducible in both 3.7.8 and 4.0.2 (used demo netbox)

The following scenario was tested:

1. Created PatchPanel1.1 with two sets of Front ports, six ports in each related to 1 rear port with six positions. (Got PatchPanel with 12 front ports and two rear ports, six positions in each)
2. Created PP2 with six front ports related to the rear port with six positions
3. Created PP3 with six front ports related to the rear port with six positions
4. Connected first rear port of PatchPanel1.1 to the rear port in PP2
5. Connected second rear port of PatchPanel1.1 to the rear port in PP3

Intermediate result:

6. Created a switch and tried to connect the first port of the switch to multiple front ports on PatchPanel1.1 (see the screenshot below)

7. Clicked create and got the following error

Double checking the code on this, the problem is here:

I don't thinkk this is really a bug (it is currently not supported to have different devices "mid-span" of a cable trace), but we should catch this assertion and report a proper error.