If a group name contains the forward slash character '/', that group will synchronize to the directory server the first time, but fail on successive synchronizations.
In GroupEntrySynchronizer.java, in populateDns(), the line ...
Name entryDn = parser.parse(entryName) ;
... will barf if entryName contains a forward slash. The above line is not executed the first time the group is synchronized to the directory server, but is executed on subsequent synchronizations.