RogueSideBand is used to simulate PGP sideband opcodes and "local data" being sent over PGP. https://github.com/slaclab/surf/blob/master/axi/simlink/src/RogueSideBand.c

If a txRemData value other than zero is statically driven to the module, then it will attempt to send that code on the first cycle after a reset. This seems to always fail with

RogueSideBand: Failed to send opcode...

For now the workaround is to avoid driving txRemData with non-zero until a few cycles after reset, but this is somewhat annoying.