I keep getting this when trading some of the Z systems at Oanda, anyone have any suggestions to why?

!Service unavailable, please try again later.
!Service unavailable, please try again later.
!Service unavailable, please try again later.
!Trade not open - 316
!Result: {"errorMessage":"Service unavailable, please try again later."}
!Service unavailable, please try again later.
!Service unavailable, please try again later.
!Service unavailable, please try again later.
!Trade not open - 323

Maybe Oanda is down. It might explain why the service is unavailable and why you should try again later.

I get these same errors but only during the weekend when the market is closed. I used to be able to suppress these messages with the weekend flag but that option was taken away. I've tried to find a way to get rid of them but so far no luck. It does not appear to have any side effects as I reported this issue some time ago.

and I assume that there are no way to open those trades at a later stage when the service is available again?

Not that I have found.