Trying to login into my AGS server 10.1 via server manager throws an error see atatched pic
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Server Manager, or ArcMap too for that matter, handles the issuance and transfer of tokens automatically the first time you authenticate by username and password. The token contains a key, which is used to validate access. The tokens are by default tied to an IP address to which they were issued. If your IP changed, you'll have an issue. These tokens can also expire and throw the same error.
To request a new token navigate to
http://yourserver:6080/arcgis/tokens in a web browser and authenticate through there. It is also possible to request a shared key which is not tied to a specific IP address. I have no idea what your specific issue is, but I hope some of this helps.