I have two somewhat similar sketches that use both HC-12 [for data collection] and a W5100-type Ethernet Shield [for Email construction and transmit]. Both sketches run fine until the code for the Ethernet Shield is invoked, which creates a connection and successfully transmits an Email. Once this happens, the HC-12 no longer receives transmits. No exceptions are raised, and I am at a loss to determine the cause of the problem. [The HC-12 is using pins 50-52.]
I have other sketches that use either the HC-12 or the Ethernet Shield, and they work fine. Before I build a custom sketch to attempt problem isolation, I thought I would simply ask the question of whether known conflicts exist. I know that many questions are resolved by posting the sketch and someone other than the author scanning it. I think that impractical in this particular case. The sketch comprises 6,000+ lines of code.
If no helpful responses are forthcoming, I will continue with problem isolation, and post again if no success is achieved.