Thank you Ankitha and Kiran for your support. It helped.
Resolution:
--We have copied ASE-15_0/lib and OCS-15_0/lib3p64 from test server, and the database server is now up without any issues.
--We couldn't find any useful trace in the logs, though - what we did so far is to stop everything, killed all SAP processes, including doing /etc/rc.d/init.d/sapinit stop and cleaning shared memory segments (using cleanipc as sybadm and ipcs -a -m and ipcrm -M as root)
--It looks like (what used to happen with older ESDs) some process handles kept some files/libraries locked.
--that is why it is very important to do memory cleaning, or even LPAR restart before database upgrade.
Regards,
Rajesh