Valid v2.1.0 Schema Json File Cannot Be Imported

Hello everyone!

Currently I am manipulating my exported collection.json Files via PowerShell. I use the ConvertFrom-Json and ConvertTo-Json commands in order to manipulate the collection more easily.
However, when I want to re-import my manipulated collection.json file to Postman, I get the following error:

Error while importing: format not recognized

Now, of course I could have destroyed the required Postman schema while manipulating the collection, but I checked against the the v2.1.0 schema using a validator, and it showed no errors.
Additionally, I can run my manipulated collection via newman without any problems.

Is anyone of you doing something similar and experienced that kind of problem?

Thanks in advance