Consolidated Platform Configuration Guide, Cisco Ios Release 15.2(4)E (Catalyst 2960-X Switches - Cisco Catalyst 2960 series Configuration Manual

This error means that an internal EEM request for memory failed.
(_cerr_sub_err = 11)
This error means that the event specification ID could not be matched when the event was being registered
or that an event detector internal event structure is corrupt.
(_cerr_sub_err = 22)
This error means that an internal EEM event detector pointer was null when it should have contained a value.
(_cerr_sub_err = 26)
This error means that the subscriber index was invalid.
(_cerr_sub_err = 54)
This error means that the event detector was unavailable.
(_cerr_sub_err = 56)
This error means that the EEM event detector that handles this request is not available.

Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)

FH_ENOSUCHESID (unknown event specification ID)
FH_ENULLPTR (event detector internal error - ptr is null)
FH_ESUBSIDXINV (invalid subscriber index)
FH_EFDUNAVAIL (connection to event detector unavailable)
FH_EFDCONNERR (event detector connection error)
unregister_counter
2021