Most domains have a static authorization code (auth code) for transferring from one registrar to another. When initiating a transfer for .EU domains, a new code is generated by the registry and only active for 40 days. This means transfers with .EU domains must be initiated within that period, or a new code will need to be generated. Each auth code request generates a new code, unless the request is made within the same 24 hour period from the initial request (this will provide the same code). After use, the code becomes invalid, and a new one would need to be requested before the domain could be transferred again.
This is a security policy with EURid, the .EU registry, and is intended to prevent fraudulent domain transfers.