Thanks Dima and Erik for your prompt answers!
It was indeed a local problem. 
Thanks to the link provided by Dima, I've found the answer by running 
SSH_AUTH_SOCK=0 ssh g...@trac.sagemath.org info
This revealed that the file 
~/.ssh/id_rsa
had wrong permissions (readable by everybody). 
This happens after the upgrade to Ubuntu 18.04: for some reason, ssh-keygen 
generated this file with the wrong permissions. 
Changing the permissions fixed the problem. 
Thank you!

Eric.

-- 
You received this message because you are subscribed to the Google Groups 
"sage-devel" group.
To unsubscribe from this group and stop receiving emails from it, send an email 
to sage-devel+unsubscr...@googlegroups.com.
To post to this group, send email to sage-devel@googlegroups.com.
Visit this group at https://groups.google.com/group/sage-devel.
For more options, visit https://groups.google.com/d/optout.

Reply via email to