I suspect this is a bug... I haven't used the new Classification Sets yet; other than to clean up over a hundred auto loaded sets with no rules that came from old hidden report suites in our system..
I'm also not terribly thrilled that this new method looks like it's going to force a lot of duplicate work, unless the old classification builder where I could add QA/Dev and Prod suites into one Rule Set... the new one looks like I can only choose a single suite.... so I will have to set up identical rules multiple times, and keep multiple in sync...
If I were you, I would reach out to Client Care... they should be able to get more information about the error on the server, and identify if this is a bug of the new feature, or if something else is happening.
Good Luck.