diff --git a/Directory.Build.props b/Directory.Build.props
index b945e8251..2987a7c9e 100644
--- a/Directory.Build.props
+++ b/Directory.Build.props
@@ -1,6 +1,7 @@
 <Project>
   <PropertyGroup Condition="'$(DOTNET_TREATWARNINGSASERRORS)' == 'true' OR '$(CI)' == 'true'">
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+    <WarningsNotAsErrors>NU1901;NU1902;NU1903;NU1904</WarningsNotAsErrors>
   </PropertyGroup>
 
   <PropertyGroup>
@@ -17,4 +18,4 @@
       <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
     </PackageReference>
   </ItemGroup>
-</Project>
\ No newline at end of file
+</Project>
diff --git a/global.json b/global.json
new file mode 100644
index 000000000..fee645a7b
--- /dev/null
+++ b/global.json
@@ -0,0 +1,7 @@
+{
+    "sdk": {
+        "version": "8.0.100",
+        "rollForward": "latestFeature",
+        "allowPrerelease": false
+    }
+}
diff --git a/src/Altinn.App.Api/Controllers/InstancesController.cs b/src/Altinn.App.Api/Controllers/InstancesController.cs
index ff1539f0a..6e964895f 100644
--- a/src/Altinn.App.Api/Controllers/InstancesController.cs
+++ b/src/Altinn.App.Api/Controllers/InstancesController.cs
@@ -1,6 +1,6 @@
 using System.Globalization;
 using System.Net;
-using System.Text.Json;
+using System.Text;
 using Altinn.App.Api.Extensions;
 using Altinn.App.Api.Helpers.Patch;
 using Altinn.App.Api.Helpers.RequestHandling;
@@ -71,7 +71,7 @@ public class InstancesController : ControllerBase
     private readonly IHostEnvironment _env;
     private readonly ModelSerializationService _serializationService;
     private readonly InternalPatchService _patchService;
-    private static readonly JsonSerializerOptions _jsonSerializerOptionsWeb = new(JsonSerializerDefaults.Web);
+
     private const long RequestSizeLimit = 2000 * 1024 * 1024;
 
     /// <summary>
@@ -1244,10 +1244,7 @@ string action
                 && instancePart.ContentType.Contains("application/json", StringComparison.Ordinal)
             )
             {
-                return System.Text.Json.JsonSerializer.Deserialize<Instance>(
-                    instancePart.Bytes,
-                    _jsonSerializerOptionsWeb
-                );
+                return JsonConvert.DeserializeObject<Instance>(Encoding.UTF8.GetString(instancePart.Bytes));
             }
         }
 
diff --git a/test/Altinn.App.Api.Tests/Controllers/InstancesController_PostNewInstance.cs b/test/Altinn.App.Api.Tests/Controllers/InstancesController_PostNewInstanceTests.cs
similarity index 91%
rename from test/Altinn.App.Api.Tests/Controllers/InstancesController_PostNewInstance.cs
rename to test/Altinn.App.Api.Tests/Controllers/InstancesController_PostNewInstanceTests.cs
index 9d8f91fb2..e931eff9b 100644
--- a/test/Altinn.App.Api.Tests/Controllers/InstancesController_PostNewInstance.cs
+++ b/test/Altinn.App.Api.Tests/Controllers/InstancesController_PostNewInstanceTests.cs
@@ -297,6 +297,51 @@ public async Task PostNewInstanceWithInstanceTemplate()
         TestData.DeleteInstanceAndData(org, app, instance.Id);
     }
 
+    [Fact]
+    public async Task PostNewInstanceWithInstanceTemplateString()
+    {
+        string org = "tdd";
+        string app = "contributer-restriction";
+        int instanceOwnerPartyId = 501337;
+        // Get an org token
+        // (to avoid issues with read status being set when initialized by normal users)
+        HttpClient client = GetRootedClient(org, app, 0, null, serviceOwnerOrg: org);
+
+        using var content = new StringContent(
+            $$"""
+            {
+                "instanceOwner": {
+                    "partyId": {{instanceOwnerPartyId}}
+                },
+                "status": {
+                    "readStatus": "UpdatedSinceLastReview",
+                    "substatus": {
+                        "label": "min label",
+                        "description": "min beskrivelse"
+                    }
+                }
+            }
+            """,
+            Encoding.UTF8,
+            "application/json"
+        );
+
+        var response = await client.PostAsync($"{org}/{app}/instances", content);
+        var responseContent = await response.Content.ReadAsStringAsync();
+        OutputHelper.WriteLine(responseContent);
+        response.Should().HaveStatusCode(HttpStatusCode.Created);
+        var instance = JsonSerializer.Deserialize<Instance>(responseContent, JsonSerializerOptions)!;
+        instance.Should().NotBeNull();
+        instance.Id.Should().NotBeNullOrEmpty();
+        instance.Status.Should().NotBeNull();
+        instance.Status.ReadStatus.Should().Be(ReadStatus.UpdatedSinceLastReview);
+        instance.Status.Substatus.Should().NotBeNull();
+        instance.Status.Substatus!.Label.Should().Be("min label");
+        instance.Status.Substatus!.Description.Should().Be("min beskrivelse");
+
+        TestData.DeleteInstanceAndData(org, app, instance.Id);
+    }
+
     [Fact]
     public async Task PostNewInstanceWithMissingTemplate()
     {
diff --git a/test/Altinn.App.Api.Tests/CustomWebApplicationFactory.cs b/test/Altinn.App.Api.Tests/CustomWebApplicationFactory.cs
index 4da6c0371..bd8c62a8f 100644
--- a/test/Altinn.App.Api.Tests/CustomWebApplicationFactory.cs
+++ b/test/Altinn.App.Api.Tests/CustomWebApplicationFactory.cs
@@ -98,10 +98,17 @@ public Task Invoke(HttpContext httpContext)
         }
     }
 
-    public HttpClient GetRootedClient(string org, string app, int userId, int? partyId, int authenticationLevel = 2)
+    public HttpClient GetRootedClient(
+        string org,
+        string app,
+        int userId,
+        int? partyId,
+        int authenticationLevel = 2,
+        string? serviceOwnerOrg = null
+    )
     {
         var client = GetRootedClient(org, app);
-        string token = PrincipalUtil.GetToken(userId, partyId, authenticationLevel);
+        string token = PrincipalUtil.GetToken(userId, partyId, authenticationLevel, org: serviceOwnerOrg);
         client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", token);
         return client;
     }
diff --git a/test/Altinn.App.Api.Tests/Mocks/InstanceClientMockSi.cs b/test/Altinn.App.Api.Tests/Mocks/InstanceClientMockSi.cs
index 4c2dc556d..fa4ca2b1f 100644
--- a/test/Altinn.App.Api.Tests/Mocks/InstanceClientMockSi.cs
+++ b/test/Altinn.App.Api.Tests/Mocks/InstanceClientMockSi.cs
@@ -21,26 +21,14 @@ public InstanceClientMockSi(ILogger<IInstanceClient> logger, IHttpContextAccesso
         _httpContextAccessor = httpContextAccessor;
     }
 
-    public Task<Instance> CreateInstance(string org, string app, Instance instanceTemplate)
+    public Task<Instance> CreateInstance(string org, string app, Instance instance)
     {
-        string partyId = instanceTemplate.InstanceOwner.PartyId;
+        string partyId = instance.InstanceOwner.PartyId;
         Guid instanceGuid = Guid.NewGuid();
-
-        Instance instance =
-            new()
-            {
-                Id = $"{partyId}/{instanceGuid}",
-                AppId = $"{org}/{app}",
-                Org = org,
-                InstanceOwner = instanceTemplate.InstanceOwner,
-                Process = instanceTemplate.Process,
-                Data = new List<DataElement>(),
-            };
-
-        if (instanceTemplate.DataValues != null)
-        {
-            instance.DataValues = instanceTemplate.DataValues;
-        }
+        instance.Id = $"{partyId}/{instanceGuid}";
+        instance.AppId = $"{org}/{app}";
+        instance.Org = org;
+        instance.Data = new List<DataElement>();
 
         string instancePath = GetInstancePath(app, org, int.Parse(partyId), instanceGuid);
         string directory =