Here are some records with failing xor checksums:
20180325-21:31:47.828 error: invalid checksum: f2 17 80 23 42 18 00 25 45 01 04 70 1d 00 00 04 40 80 50
20180325-21:31:52.351 error: invalid checksum: f2 17 80 23 42 18 00 25 49 01 04 70 25 00 00 04 08 00 64
20180325-21:32:01.463 error: invalid checksum: f2 17 80 23 42 18 00 25 45 15 02 fb 1f 00 00 04 40 00 4e
20180325-21:32:06.053 error: invalid checksum: f2 17 80 23 42 06 00 25 46 01 04 70 20 00 00 04 40 00 6e
20180325-21:32:07.741 error: invalid checksum: f2 17 80 23 42 18 00 25 40 21 02 7e 22 00 00 04 40 00 76
20180325-21:32:11.521 error: invalid checksum: f2 17 80 23 42 18 00 25 47 01 04 70 01 00 00 04 40 00 6c
20180325-21:32:20.566 error: invalid checksum: f2 17 80 23 42 18 00 25 49 15 02 7e 07 00 00 04 40 00 43
20180325-21:32:25.089 error: invalid checksum: f2 17 80 23 42 18 40 25 43 01 04 70 16 00 00 04 40 00 5d
20180325-21:32:26.844 error: invalid checksum: f2 17 80 23 42 18 00 25 a9 15 02 7e 21 00 00 04 40 00 0a
20180325-21:32:29.611 error: invalid checksum: f2 17 80 88 42 18 00 25 42 01 04 70 0c 00 00 04 40 00 46
20180325-21:32:31.366 error: invalid checksum: f2 17 80 23 42 18 00 22 42 01 04 70 0c 00 00 04 40 00 46
20180325-21:32:35.889 error: invalid checksum: f2 17 80 23 42 0c 00 25 44 01 04 70 14 00 00 04 40 00 58
20180325-21:32:48.714 error: invalid checksum: f2 17 80 23 42 18 00 25 40 01 00 70 0b 00 00 04 40 00 43
20180325-21:32:53.236 error: invalid checksum: f2 17 80 23 21 18 00 25 3e 15 02 7e 1d 00 00 04 40 00 37
20180325-21:32:54.991 error: invalid checksum: f2 17 80 23 42 18 00 25 09 01 04 70 01 00 00 04 40 00 37
20180325-21:32:57.759 error: invalid checksum: f2 17 80 23 42 01 00 25 3d 15 02 7e 20 00 00 04 40 00 09
20180325-21:32:59.446 error: invalid checksum: f2 17 80 23 10 18 00 25 3d 15 02 7e 20 00 00 04 40 00 09
20180325-21:33:09.571 error: invalid checksum: f2 17 80 23 42 18 00 25 45 15 02 7e 07 00 00 04 40 00 4e
20180325-21:33:12.339 error: invalid checksum: f2 17 80 23 42 01 00 25 43 01 04 70 16 00 00 04 40 00 5d
20180325-21:33:14.161 error: invalid checksum: f2 17 80 23 42 18 00 25 43 20 04 70 16 00 00 04 40 00 5d
I don't really understand what's happening here. It looks a bit like random bit errors. could it be a birate mismatch between CPU and BLE module?
Maybe someone with more experience with this kind of errors can comment on this.
This looks like a baudrate problem, or maybe wrong start bit detection or something like this. E.g. when 0x42 turns into 0x21, one bit shifted to the right. Same for 0x18 turning int 0x0c, one bit shifted.
Very unlikely that it has anything to do with the RF transfer from the Bluetooth module to the receiver, because as I noted earlier, BLE has a sophisticated error correction. Not just xor, but CRC, which is easy to calculate with tables, I've implemented this once myself for another product. You can verify this if you increase the distance: there shouldn't be more errors, but it gets slower, because packets gets resend, at least when I tested it with my Blue Gecko Silabs IC, which had no packet errors at all at the application level.
Maybe you can measure the RX/TX lines between the Bluetooth module and the CPU to check if the baudrate is exact in your multimeter, if David can't reproduce it?