I think I have found the error. I just ran the .desktop command in the terminal. And in the output you can see the error:
[nemo@XA2Plus ~]$ /usr/bin/sailjail -p jolla-calendar.desktop /usr/bin/jolla-cal
endar
constructing /run/firejail/mnt/privileged: Accounts,Keys,msyncd,Calendar,Contacts,Images,eas-sailfish ...
mounting /run/firejail/mnt/privileged @ /home/nemo/.local/share/system/privileged
hiding /run/firejail/mnt/privileged
Error fcopy: invalid ownership for file /etc/hosts
Error: failed to run /run/firejail/lib/fcopy, exiting...
Error: proc 18346 cannot sync with peer: unexpected EOF
Peer 18351 unexpectedly exited with status 1
What kind of owner needs the hosts?
devel-su chown -c root:root /etc/hosts
Is it enough like this?