Dynamics CRM error: "The object cannot be updated because it is read-only"

I've got an instance of Dynamics CRM 4.0 with some weird errors. If I close a Case and then try to reactivate it I get the error "The object cannot be updated because it is read-only". This is simple, built-in CRM functionality that works fine on other instances. I get the same error if I try to convert a Quote to an Order - I believe it's trying to update the Quote, which has been "closed" by then. What's wrong and how do I fix it?

MSCRM Error Report:
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Error: Exception of type 'System.Web.HttpUnhandledException' was thrown.

Error Number: 0x8004022E

Error Message: The object cannot be updated because it is read-only.

Error Details: The object cannot be updated because it is read-only.

Source File: Not available

Line Number: Not available

Request URL: http://jupiter:5555/EN/cs/cases/edit.aspx?id={C75704E2-3B82-DE11-BFAD-00188B2CB446}

Stack Trace Info: [CrmException: The object cannot be updated because it is read-only.]
   at Microsoft.Crm.ObjectModel.IncidentService.Update(IBusinessEntity incident, ExecutionContext context)

[TargetInvocationException: Exception has been thrown by the target of an invocation.]
   at System.RuntimeMethodHandle._InvokeMethodFast(Object target, Object[] arguments, SignatureStruct& sig, MethodAttributes methodAttributes, RuntimeTypeHandle typeOwner)
   at System.RuntimeMethodHandle.InvokeMethodFast(Object target, Object[] arguments, Signature sig, MethodAttributes methodAttributes, RuntimeTypeHandle typeOwner)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean skipVisibilityChecks)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at System.Web.Services.Protocols.LogicalMethodInfo.Invoke(Object target, Object[] values)
   at Microsoft.Crm.Extensibility.InternalOperationPlugin.Execute(IPluginExecutionContext context)
   at Microsoft.Crm.Extensibility.PluginStep.Execute(PipelineExecutionContext context)
   at Microsoft.Crm.Extensibility.Pipeline.Execute(PipelineExecutionContext context)
   at Microsoft.Crm.Extensibility.MessageProcessor.Execute(PipelineExecutionContext context)
   at Microsoft.Crm.Extensibility.InternalMessageDispatcher.Execute(PipelineExecutionContext context)
   at Microsoft.Crm.Extensibility.ExternalMessageDispatcher.Execute(String messageName, Int32 primaryObjectTypeCode, Int32 secondaryObjectTypeCode, PropertyBag fields, CorrelationToken correlationToken, CallerOriginToken originToken, UserAuth userAuth, Guid callerId)
   at Microsoft.Crm.Sdk.RequestBase.Process(Int32 primaryObjectTypeCode, Int32 secondaryObjectTypeCode, CorrelationToken correlationToken, CallerOriginToken originToken, UserAuth userAuth, Guid callerId)
   at Microsoft.Crm.Sdk.RequestBase.Process(CorrelationToken correlationToken, CallerOriginToken originToken, UserAuth userAuth, Guid callerId)
   at Microsoft.Crm.Sdk.CrmServiceInternal.Execute(RequestBase request, CorrelationToken correlationToken, CallerOriginToken originToken, UserAuth userAuth, Guid callerId)
   at Microsoft.Crm.Sdk.InProcessCrmService.Execute(Object request)
   at Microsoft.Crm.Application.Platform.ServiceCommands.PlatformCommand.ExecuteInternal()
   at Microsoft.Crm.Application.Platform.ServiceCommands.UpdateCommand.Execute()
   at Microsoft.Crm.Application.Platform.EntityProxy.Update(Boolean performDuplicateCheck)
   at Microsoft.Crm.Application.Platform.EntityProxy.Update()
   at Microsoft.Crm.Web.CS.CaseDetailPage.Open(Object sender, DataEventArgs e)
   at Microsoft.Crm.Application.Forms.DataEventProcessor.Raise(FormEventId eventId, FormState state, IUser user, String objectId, Entity entity)
   at Microsoft.Crm.Application.Forms.AppForm.RaiseDataEvent(FormEventId eventId)
   at Microsoft.Crm.Application.Forms.EndUserForm.Initialize(Entity entity)
   at Microsoft.Crm.Application.Forms.CustomizableForm.Execute(Entity entity, String formType)
   at Microsoft.Crm.Application.Forms.CustomizableForm.Execute(Entity entity)
   at Microsoft.Crm.Web.CS.CaseDetailPage.ConfigureForm()
   at Microsoft.Crm.Application.Controls.AppUIPage.OnPreRender(EventArgs e)
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

[HttpUnhandledException: Exception of type 'System.Web.HttpUnhandledException' was thrown.]
   at System.Web.UI.Page.HandleError(Exception e)
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest()
   at System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context)
   at System.Web.UI.Page.ProcessRequest(HttpContext context)
   at ASP.en_cs_cases_edit_aspx.ProcessRequest(HttpContext context)
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

Try disabling your onload and onsave code to see if that is causing any problems. To quickly and temporarily disable the events, open the dialog box where you paste in your onload/onsave code and uncheck 'Event is enabled'. Then save the form and publish the entity. Does the error still occur?