This has happened to me on a number of SBS boxes whit low hard drive space, generally on drive C:
SBS resets the cal count to 5!
We have just had a customer call and this has happened to. Normally we backup the CALS using the built in backup utility within SBS Console.
Found this MS KB 888818, which isn’t really much help.
Then found post on Knight-Time Ramblings which worked!