Unable to find target repo for pull request

Dear git experts,

I am trying to create a PR from a forked repo to an other fork of the same repo.
The source branch is this one [1] and the target is this one [2].

When creating the PR, the target repo does not show up in the available options. The reverse however is possible (when I switch target and destination).
Here is a screenshot of the PR attempt:

I know that in the past I have been able to make PR’s like this
(see pull 23 and 18 in the target branch pull request history, sorry I could not provide more then 2 links as I am a new user).
Am I missing something obvious?

[1] GitHub - SenneVanPutte/cmssw at newAIOtool_GCP
[2] GitHub - cms-trackeralign/cmssw at newAllInOneTool