I've investigated this and found out that we're still blocking bank transfer donations from being deleted. While implementing T276817: Add use cases for moderating and soft-deleting donations we discussed & agreed verbally that we need to change this, but never followed through.

PR is available: https://github.com/wmde/fundraising-donations/pull/154

When that is merged, we need to create a new bugfix release (e.g. 7.0.2) for the donation bounded context, update the dependencies of Fundraising Operation Center and deploy it.
The Fundraising App is not affected and does not need an immediate update and deployment (the cancel link is hidden for bank data donations).

The new application was deployed, but the servers still ran the old version. I have cleared the opcache and now it works as expected. I'll talk to the hosting provider to find a solution for our deployments.