The client has to have the same FritzBot install that the server has.
As for SV_PURE - try with it on and off. I can't reproduce the problem, but its been mentioned that sv_pure 1 can make it hard to change the map remotely using rcon.
Let me know if you have any issues using rcon with sv_pure set to 1.
By "same Fritzbot install," does that include the waypoint files and bot files, or just the dlls and bin.pk3? And I'm fairly certain that it doesn't include the actual map located in /Main, does it?
With sv_pure OFF, it wouldn't catch it if any of those files were different.