[Adjo]

This only happens because you're using sm_rcon rcon - which doesn't make much sense, and I doubt it would've worked before in the way you've shown it.

The reason I'd guess why it doesn't work is because sm_rcon and rcon both send the input to the server console to be executed, if you do sm_rcon rcon, the server doesn't know what to connect to (since there is a second 'rcon', because the server doesn't have any rcon_address set) for executing the command. If the above is true then it could be worked around, but it would be silly doing so.

If you want to load a plugin via sm_rcon, this would work: sm_rcon sm plugins load pluginpath
Likewise for just rcon (after having set the rcon_password) : rcon sm plugins load pluginpath - or, if you're using something like HLSW or any other remote administration tool, the *_rcon parts can be ignored.