Ok, I ran into this problem : http://www.howtoforge.com/forums/sho...hlight=grpconv
Fact is I messed up something copying lines in shadows and group, so there were discrepencies.
Running grpck (not grpchk as in linked thread) and pwck did the trick.
Now grpconv returns instantly with no error.
The server doesn't seem to hang anymore when saving changes from ISPConfig.
Hope it wiil be the last update of this thread.