ok, that definitely sounds like a stupid bug I need to fix.
Could you please provide the exact payload bodies for the two subscriptions.
Having that it should be straightforward to reproduce the issue and then also straightforward to fix it.
Thank you for reporting!

Thank you for your attention!

I realised that it is sufficient to have a registered subscription and then query V2 API

curl -X POST -H 'fiware-service: test' -H 'fiware-servicepath: /#' -H 'content-type: application/ld+json' -d '{
  "description": "Notify me of all product price changes",
  "type": "Subscription",
  "entities": [{"type": "Product"}],
  "watchedAttributes": ["price"],
  "notification": {
    "format": "keyValues",
    "endpoint": {
      "uri": "http://tutorial:3000/subscription/price-change",
      "accept": "application/json"
    }
  },
   "@context": "https://smartdatamodels.org/context.jsonld"
}' http://localhost:1026/ngsi-ld/v1/subscriptions`

and then:

curl -H 'fiware-service: test' http://localhost:1026/v2/subscriptions
curl: (52) Empty reply from server

I hope this helps

That should really help. Thanks!

So, I implemented a functional test for this.
Only, it doesn't fail. All good ...

I used mongo v3.6 and what I have in the develop branch right now. No error.
I also tried the version 1.6.0 of Orion-LD. Also no problem.
So, not able to reproduce.
Need more input.
Please supply the dockerfile you use for Orion-LD and I'll start my Orion-LD with the exact same parameters.
In my test I used only:

-multiservice [-experimental]

(I tried both with and without the -experimental CLI)

Forget the comment about not being able to reproduce.
I did the GET /subscriptions using LD and not V2.
Once I realized my mistake, I reproduced the crash immediately, and in a few minutes I fixed it.
PR on its way, should be ready in an hour or so.