You can find the error code definitions in cyu3error.h
I suppose the 64 value returned is in decimal (not in hex), please check the debug print function whether it prints %d or %x in your case. 64 in decimal is 0x40 in hex which corresponds to "CY_U3P_ERROR_BAD_ARGUMENT" /**< One or more parameters to a function are invalid. */
Thank you, I hadn't noticed that some numbers in the error list were skipped, and had miscounted. The problem is resolved.