Here's my guess about the problem :
I forgot to upgrade my installation before dumping my database. So, on the new server, I had a 2.2.29 install with a 2.2.25 database.
I upgraded the new install from 2.2.29 to 2.2.30 and hope it will do the trick. The process went fine.
Just after the upgrade, a process called grpconv ran and ate as much as 70% memory. I wonder if this process tried to run when I saved changes in ISPC. It would partly explain the hangs.
I hope I solved the problem with the upgrade, but I would like to read if somebody more competent than me could propose an explanation.