You are here on a von Neumann machine which has different instructions when accessing data in flash ("OK") and data in sram (rx_buff). For some library routines there were replacements which took that into account. They would be named cstrstr() or strcstr(), but I do not know whether they have been implemented for PSoC1.
A good tip for you is really: get a CY8CKIT-043 and switch to a modern ARM CPU.
Thank you, Bob!! Yeah, I have been developing on PSoC 5LP (CY8CKIT-059) and when some partner asked me to develop on PSoC 1 it was a huge change, and yes, switching to a modern ARM CPU it's a great tip. Again, thank you for your answer and time.