Hi @Koen G Thank you for your patience in handling this issue and I apologize for the inconvenience. A fix for this issue has been deployed in all the impacted regions. You should be able to make the changes on the portal for the custom connector. I appreciate it if you verify this from your end and let us know if you still experience this issue.
If the response helped, please do click Accept Answer and Yes for the answer provided. Doing so would help other community members with similar issue identify the solution. I highly appreciate your contribution to the community.