I tried updating to 6.0 preview 7, razden 3.9.2, system.linq.dynamic.core 1.2.12. I can select an item in the drop down control but searching still gives an error (below). Grid control sorting and filtering seems to work though.
Microsoft.AspNetCore.Components.WebAssembly.Rendering.WebAssemblyRenderer[100]
Unhandled exception rendering component: The type initializer for 'System.Linq.Dynamic.Core.DynamicQueryableExtensions' threw an exception.
System.TypeInitializationException: The type initializer for 'System.Linq.Dynamic.Core.DynamicQueryableExtensions' threw an exception.
---> System.Exception: Specific method not found: FirstOrDefault
---> System.InvalidOperationException: Sequence contains more than one matching element
at System.Linq.ThrowHelper.ThrowMoreThanOneMatchException() in System.Linq.dll:token 0x6000319+0x0
at System.Linq.Enumerable.TryGetSingle[MethodInfo](IEnumerable`1 source, Func`2 predicate, Boolean& found) in System.Linq.dll:token 0x60000d3+0x3d
at System.Linq.Enumerable.Single[MethodInfo](IEnumerable`1 source, Func`2 predicate) in System.Linq.dll:token 0x60000cd+0x0
at System.Linq.Dynamic.Core.DynamicQueryableExtensions.GetMethod(String name, Int32 parameterCount, Func`2 predicate) in System.Linq.Dynamic.Core.dll:token 0x60000d1+0x14
--- End of inner exception stack trace ---
at System.Linq.Dynamic.Core.DynamicQueryableExtensions.GetMethod(String name, Int32 parameterCount, Func`2 predicate) in System.Linq.Dynamic.Core.dll:token 0x60000d1+0x3e
at System.Linq.Dynamic.Core.DynamicQueryableExtensions..cctor() in System.Linq.Dynamic.Core.dll:token 0x60000d2+0xe3
--- End of inner exception stack trace ---
at Radzen.DropDownBase`1[[System.Nullable`1[[System.Int64, System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]], System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].get_View() in Radzen.Blazor.dll:token 0x60001f9+0x92
at Radzen.DropDownBase`1.<HandleKeyPress>d__64[[System.Nullable`1[[System.Int64, System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]], System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].MoveNext() in Radzen.Blazor.dll:token 0x60012c5+0x41
at Radzen.DropDownBase`1.<OnFilterKeyPress>d__65[[System.Nullable`1[[System.Int64, System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]], System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].MoveNext() in Radzen.Blazor.dll:token 0x60012c7+0x69
at Microsoft.AspNetCore.Components.ComponentBase.CallStateHasChangedOnAsyncCompletion(Task task) in Microsoft.AspNetCore.Components.dll:token 0x60000a5+0x7e
at Radzen.DropDownBase`1[[System.Nullable`1[[System.Int64, System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]], System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].get_View() in Radzen.Blazor.dll:token 0x60001f9+0x92
at Radzen.Blazor.RadzenDropDown`1[[System.Nullable`1[[System.Int64, System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]], System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].BuildRenderTree(RenderTreeBuilder __builder) in Radzen.Blazor.dll:token 0x600088c+0xb3d
at Microsoft.AspNetCore.Components.ComponentBase.<.ctor>b__6_0(RenderTreeBuilder builder) in Microsoft.AspNetCore.Components.dll:token 0x60000a8+0xe
at Microsoft.AspNetCore.Components.Rendering.ComponentState.RenderIntoBatch(RenderBatchBuilder batchBuilder, RenderFragment renderFragment, Exception& renderFragmentException) in Microsoft.AspNetCore.Components.dll:token 0x600030a+0x17