>>4 ETW000 [dbcon.c ,00000] *** ERROR => Version mismatch between executable (742.06) and DBSL shared library (721.02)<<
I believe you updated the kernel in the <SUM Directory>/abap/exe directory and try to test the DB connection using the R3trans from the <SUM Directory>/abap/exe directory.
If yes R3trans will use the DBSL from the /usr/sap/PSB/SYS/exe/run location because of the environment variables of the <SID>adm user.
This is the reason why you find a mismatch in the kernel files.
If you are in the middle of the upgrade and if you have updated the kernel for the shadow system then start the shadow system to see if the system comes up.
RB