I had to set a static port for mountd in /etc/nfs.conf and also had to open port 111 (PortMapper) and the mountd port in the cloudron firewall. rpcinfo -p helped me with this. Now I can access my nfs server.
btw: Your guide for the nfs share (https://docs.cloudron.io/guides/nfs-share/) is outdated. Since ubuntu 22.04 you have to deactivate NFSv3 in /etc/nfs.conf.
https://lansible.com/2023/10/disable-nfsv3-on-ubuntu-22.04-and-newer/
Thanks!