Answer
This happens when the license being applied doesn't cover the number of users currently active in the system — typically either because the new license has been downgraded to fewer named users than are presently set active, or because an excessive number of user accounts were created by some process/tool that bypassed IFS's own license-control checks. The fix is either side of that equation: obtain/apply a license that covers the actual current user count, or reduce the active user count by setting some users to inactive (via user/group administration in Solution Manager) until it fits within the license being applied.