Answer
Two things to check: first, the Oracle Client's architecture (32-bit vs 64-bit) must exactly match the installed Crystal Reports application's own architecture — a 32-bit Crystal Reports Developer needs a 32-bit Oracle Client, and a 64-bit Crystal install needs a 64-bit client; a mismatch here causes exactly this kind of silent connection failure in Crystal while other tools still connect fine. Second, check which specific Oracle Client type was installed — an 'Instant Client' installation doesn't create the full folder structure (including the network/admin folder that holds tnsnames.ora) the way a 'Runtime' or 'Administrator' client installation does. If that folder structure is missing, reinstalling the Oracle Client as Runtime or Administrator type creates the proper folders plus the tools needed to create and edit tnsnames.ora directly, which Crystal's Database Expert then relies on.