ad failed to load code: 0, cause: null

[David Moss]

after loading 5 or 6 ads, of any type, cumulative across the app and having them pending, unwatched, i get ad failed to load error with the following error:

Failed to load ad for ad unit ID: ca-app-pub-3940256099942544/5224354917, Error: {
  "Code": 0,
  "Message": "Internal error.",
  "Domain": "com.google.android.gms.ads",
  "Cause": "null",
  "Response Info": {
    "Response ID": "YMhnZtniG6K5jvQPjvOMwAM",
    "Mediation Adapter Class Name": "",
    "Adapter Responses": [
      {
        "Adapter": "com.google.ads.mediation.admob.AdMobAdapter",
        "Latency": 60001,
        "Ad Source Name": "Reservation campaign",
        "Ad Source ID": "7068401028668408324",
        "Ad Source Instance Name": "[DO NOT EDIT] Publisher Test Video Campaign",
        "Ad Source Instance ID": "6877926149710223",
        "Credentials": {
          "pubid": "ca-app-pub-3940256099942544\/5224354917\/cak=no_cache&cadc=43&caqid=YMhnZp7MGobBjvQP2oOp4A4",
          "campaign_type": "1",
          "campaign_id": "1057492276"
        },
        "Ad Error": {
          "Code": 0,
          "Message": "Internal error.",
          "Domain": "com.google.android.gms.ads",
          "Cause": "null"
        }
      }
    ],
    "Response Extras": {
      "mediation_group_name": "Campaign"
    }
  }
}

I ended up having to set a variable to track how many ads are loaded of each type, to prevent it from loading more than 5 total at a time. this works, and fortunately my app currently has only 5 ad id's, so i preload the next one while they're watching in the background, but if i had 6 ad id's, right now i'd have to keep one unloaded which means they couldn't watch that ad.

for testing, i use the test ad id as seen in the log there, and if it loads 5, i use one, then it loads the next one, it works. if i however tell it to load 10, it fails at 5 or 6, inconsistently, and always with this error

any thoughts or help would be appreciated.