This one here happened on Oanda USA. Every time that a trade would start, I would get several messages in my MT4 inbox stating: "First In First Out Violation". I could not find anything at all wrong other than I had a "Pass" block right above the "No Trade" block. I am not sure what the "Pass" block really does but it technically did not have to be there. I reconfigured EA leaving the "Pass" block out. After I reloaded the EA into the MT4, no more error messages in my mailbox upon order fill.
So, for this error or any other error: If there are blocks that don't need to be there: take them out. It helps in some cases. Instructions also mention it.
FIFO rule only applies to those based in USA and trade with a USA regulated broker. It's a pain to deal with but it has been around for over a decade so I don't see it going away. It's just another example of rules being made up by someone who has never traded. Very typical.