The concurrent manager is working fine now. I had to recreate the network 8.0.6 environment (make -f ins_network.mk install), since the lsnrctl executable (amongst others) did not exist.
This recreation of executables gave a terrible list of link errors (undefined reference to `__ctype_b'). After a long search, I found out (www.puschitz.com) that I had to downgrade the glibc libraries to glibc-2.3.2-5.i686.rpm. Once this was done, the make finished well, and up was the concurrent manager.