Mitel seem to know about the problem, found this on MOL:
When adding additional SIP, IP user or Voicemail licenses via ESM on an existing system running MCD 4.1, system may return with "internal system error" when the "SAVE" button is pressed in License and Option Selections.
Analysis
During "Save" operation, ESM collects the existing licenses from 3300 and compares the new licenses to be added.
3300 returns an improper device license to ESM. "Internal System Error" is then displayed on ESM.
Temporary Solution:
Option A: Use Software Installer ( SI ) to re-license an existing MCD 4.1 system
Note: SI will perform system reboot after relicencing.
Option B: Perform Full Installation
Long Term Solution
The issue is being tracked by defect 336123 and will be addressed in MCD 4.1 SP2
MCD 4.1 SP2 is scheduled to release in the first week of July, 2010