ORA-24408: could not generate unique server group name

You can downgrade to 10g if you want (it evidently doesn't care about this) but that's not necessary...

The 11g instant client requires a /etc/hosts file entry for your hostname pointing to 127.0.0.1. The normal "localhost" entry is not sufficient on it's own.

Assuming your host name is foomachine, there are two places you'll need to check:

In /etc/hosts, make sure you have any entry like - add it if it's not there:

127.0.0.1   foomachine

And also make sure the /etc/sysconfig/network file also has HOSTNAME=foomachine

That should do the trick.

Kind of an old question, but I just stumbled on it after upgrading my Mac to Sierra. Until then I hadn't had the problem.

The trick is definitely to add your hostname to /etc/hosts, but you have to have the right hostname. That's easy on linux, it's in /etc/sysconfig/network. On Mac find it in System Preferences > Sharing. Under where it says Computer name will be something like computer-name.local. In /etc/hosts put

   127.0.0.1 computer-name.local

Replacing computer-name with your computer name of course :). It needs this regardless of whether the database is a remote one or a local one.

I had plenty of other aliases for 127.0.0.1 but not that one. As soon as I added that one my apps started working again.

In my situation, the OracleDB server I was trying to connect to was a remote one, not a local one, so the above 127.0.0.1 localhost trick didn't work.

Previously, I had temporarily fixed a DNS problem by adding an entry for my Oracle DB server into my hosts file.This IP in my hosts file was still correct - it still pointed to the correct OracleDB server IP.

By removing the entry from my hosts file, the problem went away.

I have no idea why this is a problem for OracleDB, or why the error message is so obtuse, but I hope this helps someone else.

(This was using InstantClient v12.1.0.2 on MacOS v10.11.5)