SBS 4.5 License Service Error ID:201

I have a Small Business Server (NT4 with Exchange 5.5 and SP6) Version 4.5 with 25 CAL. For some reason now the server will allow only a max 20 Exchange users. I should have the same number of licenses for Exchange as I do for the server. The server originally started out with 5 CAL and we have been adding CALs in increments of 5 over the past two years. Now my NT and Exchange licenses are not in sync. I reinstalled SP6. That did not help.

Event log:
Source: LicenseService
Event ID: 201
Description: No license was available for user ‘user_name’ using product Exchange.