Unable to view Optimizely Forms submissions when some values are too long

Recently, a customer had a form using Optimizely Forms that worked without problems.

Suddenly, it was no longer possible to view form submissions using the menu item shown below.

Upon investigating, I found that one submission had some very long values (>500KB) and I observed a 500 error in my browser's developer tools.

In the logs, I found the following:

fail: Microsoft.AspNetCore.Diagnostics.ExceptionHandlerMiddleware[1]
An unhandled exception has occurred while executing the request.

System.InvalidOperationException: Synchronous operations are disallowed. Call WriteAsync or set AllowSynchronousIO to true instead.
   at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpResponseStream.Write(Byte[] buffer, Int32 offset, Int32 count)
   at System.IO.Compression.DeflateStream.WriteDeflaterOutput()
   at System.IO.Compression.DeflateStream.WriteCore(ReadOnlySpan`1 buffer)
   at System.IO.Compression.DeflateStream.Write(Byte[] buffer, Int32 offset, Int32 count)
   at Microsoft.AspNetCore.ResponseCompression.ResponseCompressionBody.Write(Byte[] buffer, Int32 offset, Int32 count)
   at Microsoft.AspNetCore.WebUtilities.HttpResponseStreamWriter.Write(String value)
   at Newtonsoft.Json.Utilities.JavaScriptUtils.WriteEscapedJavaScriptString(TextWriter writer, String s, Char delimiter, Boolean appendDelimiters, Boolean[] charEscapeFlags, StringEscapeHandling stringEscapeHandling, IArrayPool`1 bufferPool, Char[]& writeBuffer)
   at Newtonsoft.Json.JsonWriter.WriteValue(JsonWriter writer, PrimitiveTypeCode typeCode, Object value)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeDictionary(JsonWriter writer, IDictionary values, JsonDictionaryContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeList(JsonWriter writer, IEnumerable values, JsonArrayContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.Serialize(JsonWriter jsonWriter, Object value, Type objectType)
   at Newtonsoft.Json.JsonSerializer.SerializeInternal(JsonWriter jsonWriter, Object value, Type objectType)
   at Newtonsoft.Json.JsonSerializer.Serialize(JsonWriter jsonWriter, Object value)
   at Newtonsoft.Json.JsonSerializer.Serialize(TextWriter textWriter, Object value)
   at EPiServer.Formatters.Internal.ExtendedNewtonsoftJsonOutputFormatter.WriteResponseBodyAsync(OutputFormatterWriteContext context, ResponseDecorator responseDecorator)
   at EPiServer.Formatters.Internal.ExtendedNewtonsoftJsonOutputFormatter.WriteAsync(OutputFormatterWriteContext context, ResponseDecorator responseDecorator)
   at EPiServer.Shell.Services.Rest.RestResultBase.ExecuteResultAsync(ActionContext context)
   at EPiServer.Shell.Services.Rest.RestResult.ExecuteResultAsync(ActionContext context)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeResultAsync>g__Logged|22_0(ResourceInvoker invoker, IActionResult result)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeNextResultFilterAsync>g__Awaited|30_0[TFilter,TFilterAsync](ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Rethrow(ResultExecutedContextSealed context)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.ResultNext[TFilter,TFilterAsync](State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.InvokeResultFilters()

After talking to the helpful people at Optimizely support, I added the following code snippet inside ConfigureServices() in startup.cs, for when the code runs in DXP:

services.Configure<KestrelServerOptions>(options => { options.AllowSynchronousIO = true; });

This resolved the issue!