Check the sqlnet.ora file for names.default_domain. If that entry is present (company.com, for example), then your tnsnames.ora entry must be cimdb.company.com, not just cimdb.

My guess is that your setting is ZA.TISCALI.COM, which is why a connection to DEV.ZA.TISCALI.COM would work, but one to CIMDB.ZA.TISCALI.COM would not because all you have is CIMDB in the tnsnames.ora file. Plain and simple, that's what it is.