It finally finished..
While I was waiting, I looked at pvecm status and found out it thought I had 4 nodes (see expected votes), when I only have 2 nodes.
I used pvecm expected 2 on both and suddenly the updates moved on and I was able to log in to the web interface immediately.
In the web interface, it showed a pve3 and pve4. These were some test setups I did months ago.
I deleted the nodes from the CLI and everything looks clean again.