Hello all,

In the 2026 Q1 release we refactored the serialization routines in ASP.NET Core REST Service, replacing the dependency to NewtonsoftJson with System.Text.Json.

You can read more details in the links below:

• Product upgrade path: 2026 Q1 (20.0.26.211) Changes and Dependencies.
• Public feedback item: Migrate to System.Text.Json for serialization, instead of using Newtonsoft.Json.
• Product documentation: Use Both 'System.Text.Json' and 'Newtonsoft.Json' for Serialization in ASP.NET Core.

Hello,

I suspect that the issue is experienced after upgrade to .NET 5. Our packages for .NET 5.0 are expected to be available in the R1 2021 Release of Reporting that is coming in the middle of January.  I would recommend voting for  .Net 5.0 support of Desktop ReportViewers and Reports generation WEB service feature request, so in this way, you will receive a notification when the feature is released.

Basically, the error message you observed is very familiar. For that reason, you may test the solution described in JSONResult.SerializerSettings Must Be of Type System.Text.Json.JsonSerializerOptions KB article.

Another approach that you can test is the one given by one of our customers in .Net Core RC2 Json problem forum post.

Regards,
Neli
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