When working with any product I typically like to go away from using default naming conventions and in this case the Avamar “Default Virtual Machine Group” was no exception.
I added half a dozen Virtual Machines to the default group and performed backup and recovery testing without any issues. Later in the week I decided to rename the group to a more meaningful name and then found that I could no longer add additional Virtual Machines to the group and received the following error.
Event Code 22524
Category: APPLICATION
Sev: PROCESS
Summary: Failed client add
After renaming the group back to the default name, everything came right and I was able to add additional Virtual Machines.
When I raised a support case, EMC support come back to me saying this was a known but undocumented limitation and would be fixed in a future version of Avamar.
Untill this is fixed I will just create new Virtual Machine groups using my desired naming conventions and leave the Default Virtual Machine Group empty and disabled.

[...] Avamar and VMware Site Recovery Manager with EMC CLARiiON arrays. The issue with Avamar involves renaming the Default Virtual Machine Group and is a known issue that will be addressed in a future release; for now, the workaround (as also [...]