I followed the instructions exactly, used them on the same bitcoin version from 2015. The only thing I changed is pulling up newer boost libraries when compiling. They compiled perfectly fine into exe files.
I run bitcoind, then do the usual bitcoincli sendrawtransaction and still get this error, despite the changed code.
The bitcoind connects just fine to nodes, the only thing that can be is maybe the node, not the bitcoincli returning this error. I will try to run my own node on a different PC, connect and check tomorrow.