This same thing happened to me today. Twice I tried running the Feature Class to Geodatabase tool in ArcCatalog (10.5.1). The Windows pop-up stated that the process had run, but there was no output and, upon digging into the Geoprocessing Results window, no messages from the tool at all (completely blank: no tool start time, end time, parameters, etc.)
Indeed, disabling background geoprocessing resolved the issue (Geoprocessing> Geoprocessing Options > uncheck the "Enable" under Background Geoprocessing).
I'd still consider this a bug behaviorally, since the software apparently doesn't realize that it's not working.