Hello Oliver,
Is the server a system hosting the ArcGIS License Server Administrator, and issue is not just to deauthorize a Single Use seat from running ArcGIS Administrator -> Support Operations?
Are the licenses to be deauthorized Single Seat licenses managed from the server, or are they concurrent seats?
The Error 7466 issue can occur with either Single Use seats or Concurrent use seats as the license deauthorization uses the same mechanism on Esri's FlexNet Publisher based licensing servers.
Unfortunately this is something that you can not resolve without Esri's assistance, you will need to contact an Esri Customer Service Representative, or your Esri distributor to assist with "deauthorizing and rearming" of your set of authorization numbers.
The details of how Esri generates the Error 7466 are a little unclear, but please review Esri KB 38753 - 7466: Deauthorization of the license is denied because max number of returns is exceeded, and also Esri KB 40627 - Selectively deauthorize licenses in ArcGIS License Server Administrator
And, if you reuse the same server (or reauthorize a Single Use seat) you would have to manually clear the FlexNet Publisher Trusted Storage cache -- that is explained in KB 40611 - Clear or delete trusted storage for concurrent-use and single-use licenses
Stuart
