This KB explains the meaning of below log

[LOG: Info] failed socket_connect, slot 0, err=socket closed

This syslog message can be seen in FPC shell logs when the inter FPC comm between FPCs with or without IFCM feature.

Inter FPC communication through the fabric is managed by a socket connection between individual FPCs.

[Sep 26 15:25:40.656 LOG: Info] failed socket_connect, slot 0, err=socket closed

[Sep 26 15:25:44.660 LOG: Info] failed socket_connect, slot 10, err=socket closed

[Sep 26 15:25:48.682 LOG: Info] failed socket_connect, slot 11, err=socket closed

[Sep 26 15:26:00.662 LOG: Info] failed socket_connect, slot 0, err=socket closed

[Sep 26 15:26:04.663 LOG: Info] failed socket_connect, slot 10, err=socket closed

[Sep 26 15:26:08.668 LOG: Info] failed socket_connect, slot 11, err=socket closed

When fpc with IFCM feature tries to establish connection with one without IFCM, it fails and results in continuous logs.

This messages are cause for FPC HW issue, if all line cards supports IFCM.
However, when you mix UKERN and AFT LC in a single chassis, you will see the error message on UKERN, which is totally harmless/expected.