Just an update on this:
I switched everything to a desktop with a dedicated ethernet port, and connected all 4 USRPs to a switch, which is then directly connected via ethernet to the computer. This has eliminated the Us, which means that it is actually working decently (and not crapping out). Thanks to all who advised with that. However, multiple Ss still get printed out at basically every transmission. In addition, there are sometimes warnings that say:
SSSSSSSSSSS
UHD Error:
Control packet attempt 0, sequence number 452:
RuntimeError: no control response, possible packet loss
This happens occasionally, but the Ss get printed out every time. While it isn't blocking, since I am actually getting transmissions and it is not crashing, I am still curious about how to solve this S problem. Thanks again!