If you receive this error while makin remote connections to a database. 

[oracle@oel71101 DMAin]$ sqlplus sys/'oracle'@ole711.internal.net:1977/ICC11DMA as sysdba

SQL*Plus: Release 12.1.0.2.0 Production on Wed Dec 1 10:04:12 2021

Copyright (c) 1982, 2014, Oracle.  All rights reserved.

ERROR:
ORA-12537: TNS:connection closed

its is possible that there is a permisison issue on oracle binary. there could be other issues as well as explained in

ORA-12537 / ORA-12547 or TNS-12518 if Listener (including SCAN Listener) and Database are Owned by Different OS User (Doc ID 1069517.1)

listner log shows that connection reached to the target database but could not handoff

 <txt>01-DEC-2021 09:25:47 * (CONNECT_DATA=(SERVICE_NAME=ICC11CU5)(CID=(PROGRAM=sqlplus)(HOST=oel71101)(USER=oracle))(SERVER=dedicated)(INSTANCE_NAME=ICC11CU5_1)) * (ADDRESS=(PROTOCOL=tcp)(HOST=10.97.85.101)(PORT=19543)) * establish * ICC11CU5 * 12518
 </txt>
</msg>
<msg time='2021-12-01T09:25:47.697+00:00' org_id='oracle' comp_id='tnslsnr'
 type='UNKNOWN' level='16' host_id='ole71101'
 host_addr='10.200.69.51' pid='31412'>
 <txt>TNS-12518: TNS:listener could not hand off client connection
 TNS-12547: TNS:lost contact
  TNS-12560: TNS:protocol adapter error
   TNS-00517: Lost contact
    Linux Error: 32: Broken pipe
 </txt>
</msg>

in my case permissions on oracle binary were

[oracle@ole71101 bin]$ ls -ltr /u01/oracle/product/12.1/db_1/bin/oracle
-rwxr-s--x. 1 oracle dba 324511576 Oct 25 09:40 /u01/oracle/product/12.1/db_1/bin/oracle

It should be “-rwsr-s–x” – 6751. So i changed it

[oracle@ole71101 bin]$ chmod 6751 /u01/oracle/product/12.1/db_1/bin/oracle
[oracle@ole71101 bin]$  ls -ltr /u01/oracle/product/12.1/db_1/bin/oracle
-rwsr-s--x. 1 oracle dba 324511576 Oct 25 09:40 /u01/oracle/product/12.1/db_1/bin/oracle

now remote connection works fine

[oracle@oel71101 ~]$ sqlplus sys/'oracle'@ole711.internal.net:1977/ICC11CU5 as sysdba

SQL*Plus: Release 12.1.0.2.0 Production on Wed Dec 1 10:52:39 2021

Copyright (c) 1982, 2014, Oracle.  All rights reserved.


Connected to:
Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit Production
With the Partitioning, Real Application Clusters, Automatic Storage Management, OLAP,
Advanced Analytics and Real Application Testing options