The ALLOC_ENGINE_V4_ALLOC_FAIL message
An "ALLOC_ENGINE_V4_ALLOC_FAIL" can be generated for a number of reason.
It would be helpfull and speed up problem resolution that when looking though log files that a more specific error messages or extention to this messages is generated.
Lease allocation timed out
No free leases on subnet X. Free leases but not available to this client X