HTTP 503 would be "Service Unavailable", so some part of the application is unavailable.
Your errors are talking about Tests so I'm guessing it is related to an app?
I suggest to either contact the app vendor (if you have a test management app installed) or Atlassian directly as you are on premium to verify your environment is still up and running correctly.
My guess would be the app but you never know for sure.
Sure, through the marketplace they list their support portal as:
Since Cloud apps are hosted with the vendor and you are getting a 503, that's my main guess.. If the rest of your instance still works fine my gut says the app :)
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