The upcoming_bytes counter indicates data that the Klipper host knows it will need to eventually send to the mcu, but also knows it shouldn’t yet send to the mcu. It is not an indication of an error; it is normal for it to be non-zero. In contrast, a very high (eg, more than 64) value in ready_bytes indicates an issue with communications - this is the counter of bytes that the host would like to send to the mcu, but is unable to due to delays on the communication line.
As to the error reported here - the Klipper host code has been heavily modified (with various ercf and other modifications), so it’s difficult to guess what the host code was attempting at the time of the failure. It does appear the host got overloaded, but why is not clear.
Cheers,
-Kevin