RF interference following me

I’ve been on a shoot out in the countryside in Portugal, using lectrosonic 400 series for wireless mics. One is block 470 and the other is block 20. Through the whole shoot, I’ll do a scan with the block 20 receiver, tune to some free space, only to find that when I tune the transmitter all of a sudden there’s RF interference in that spot. Ok, weird, do another scan and yep there’s now some blocks showing up right where I just tuned it to. Well let’s tune it to some free space and low and behold the same thing happens. The block 470 system has been working perfectly.

Does anybody know what’s going on here? I’m pretty perplexed.

The rest of the chain: cos-11D mic -> um400a -> UCR411a -> F8n Pro

The receiver is on bullwhips, one of them was cut during its previous ownership but only by a few cm.