SB2 error 0x45 Invalid .ebl header signature.

Hi All,
I am trying to get an API mode mesh network going on arduino, but my Coordinator gets error: “0x45 Invalid .ebl header signature” when it receives data.

My Setup:
XBee Pro SB2 (XB24BZ7 PIT-005-revE)

One XBee set as a Co-Ordinator, Firmware
Set defaults, then changed:
ID (PanId) = BAA2
AP (API Enable) = 2

One XBee set as a End Device, Firmware 29A7
Set defaults, then changed:
ID (PanId) = BAA2
AP (API Enable) = 2

I am using Arduino Nanos, with 3.3V and Gnd connected, and DOUT/DIN to Rx/Tx.

I am using the library, and have loaded the Series2_Tx onto the end device and Series2_Rx onto the Co-Ordinator. I changed the Tx to send to the Co-Ordinator by replacing the line:
XBeeAddress64 addr64 = XBeeAddress64(0x00000000, 0x00000000);

The Tx sends data but hits the else block which says:
// local XBee did not provide a timely TX Status Response – should not happen

The Rx receives a ZBRxResponse but .getOption() gives me 0x45.
This isn’t defined in the library, but from the documentation I find that it means:
0x45 Invalid .ebl header signature.

Going back to XCTU I can connect the Co-Ordinator and click the graph button and see the End Device appear as a remote device. I can sometimes double click it and get device details remotely, sometimes not. The Co-Ordinators console shows data in and out (I leave the Tx running on the arduino sending.)

Thanks for any help / pointers where I can research further.

Update: With arduino re-uploads, and plugging the xbees into and out of XCTU, but no code/config changes, I am now seeing 0x41
No .ebl header was received when expected.