After having included the IBM's z/OS Explorer library in the LINKLST (prefix.SFEKAUTH) an OPSOSF server is looping on the module IKJEESX0.
OPS/MVS
The module involved is an exit of the TSO SEND command named IKJEESX0 which is delivered with the IBM z/OS Explorers product (FEKEESX0).
From IBM documentation:
"TSO SEND command
The TSO SEND command can be enhanced with the IKJEESX0 user exit so it can send messages to z/OS® Explorer users. The IKJEESX0 user exit that is provided by z/OS Explorer supports a subset of the normal TSO SEND options.
{SEND | SE} 'message' USER({* | userid [userid …]}) [{NOW | LOGON | SAVE}]
The IKJEESX0 user exit is in FEK.SFEKAUTH, unless you used a different high-level qualifier during the SMP/E install of z/OS Explorer.
The only required configuration is making the exit available to TSO using STEPLIB, LINKLIST, or LPA."
The information above was taken from this link:
https://www.ibm.com/docs/en/explorer-for-zos/3.1.1?topic=message-tso-send-command
There is an IBM APAR that mentions a defect that sounds similar to the symptom you observed. The IBM APAR is PH08404 and can be found in the link below:
https://www.ibm.com/support/pages/apar/PH08404
Among the defects mentioned in this APAR we can see:
"08.z/OS Explorer TSO SEND exit FEKEESX0 (alias IKJEESX0) can go into an endless loop"