Answer
Several distinct root causes can produce this same generic error, worth checking in turn: the Crystal Web Service prerequisites weren't installed correctly; the tnsname referenced in the Crystal Web Service's own config XML is missing, doesn't match the actual entry in tnsnames.ora, or points at the wrong database; the passwords stored in the config XML are incorrect; a bit-rate (32-bit vs 64-bit) mismatch between the installed Oracle client and the Crystal .NET runtime, or having BOTH bit-rate versions of the .NET runtime installed at once, when only one should be installed; the Oracle Client Runtime itself isn't installed; if the problem is isolated to just one specific report rather than every report, the issue is likely with that specific report rather than the Crystal Web Service setup itself; or Java isn't installed, or isn't correctly referenced in the system PATH.