Plugin throwing Generic SQL Error

I'm writing a plugin for CRM Online (2013) which will write a code to a field to allow some de-duplication. The plugin is running in async (I have tried sync/post and get the same error). The step is Update of Contact.

The error seems to be a bit intermittent. When the record is updated directly the plugin runs fine. When I run a workflow against a number of contacts it fails for some records and works for others, but there seems to be no consistency over which records it will fail against.

Here is my code and it is failing on "service.update(contact)".

string email = "";
string code = "";
ColumnSet conCols = new ColumnSet("lastname", "emailaddress1", "c20_dedupecode", "c20_updatededupecode");
Entity contact = service.Retrieve("contact", targetEntity.Id, conCols);
if (!(bool)contact["c20_updatededupecode"] && context.Depth != 1)
return;// throw new Exception("1");

tracer.Trace("depth: " + context.Depth.ToString());
tracer.Trace("update code? " + contact["c20_updatededupecode"].ToString());

code = targetEntity.Contains("lastname") ? targetEntity["lastname"].ToString() : "";
tracer.Trace("build code");
if (code == "" && contact.Contains("lastname"))
code = contact["lastname"].ToString();
if (code == "")
throw new Exception("Contact must contain last name");

email = targetEntity.Contains("emailaddress1") ? targetEntity["emailaddress1"].ToString() : "";
if (email == "" && contact.Contains("emailaddress1"))
email = contact["emailaddress1"].ToString();
code += email;

code = Regex.Replace(code, "[^a-zA-Z0-9]", "");

contact["c20_dedupecode"] = code;
contact["c20_updatededupecode"] = false;
tracer.Trace("update contact");
service.Update(contact);
tracer.Trace("contact updated");

Full error trace:

Plugin Trace:

[ExportaDedupeCode: ExportaDedupeCode.ExportaDedupeCodeClass]
[6e8fcb4c-b977-e311-8462-6c3be5bedcf4: ExportaDedupeCode.ExportaDedupeCodeClass: Update of contact]

ENTER: ExportaDedupeCode.ExportaDedupeCodeClass.Execute
0
update
contact
depth: 2
update code? True
build code
update contact



Error Message:

Unhandled Exception: System.ServiceModel.FaultException`1[[Microsoft.Xrm.Sdk.OrganizationServiceFault, Microsoft.Xrm.Sdk, Version=6.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35]]: An error occurred in the ExportaDedupeCode.ExportaDedupeCodeClass plug-in:Generic SQL error.

Server stack trace: 
   at System.ServiceModel.Channels.ServiceChannel.HandleReply(ProxyOperationRuntime operation, ProxyRpc& rpc)
   at System.ServiceModel.Channels.ServiceChannel.Call(String action, Boolean oneway, ProxyOperationRuntime operation, Object[] ins, Object[] outs, TimeSpan timeout)
   at System.ServiceModel.Channels.ServiceChannelProxy.InvokeService(IMethodCallMessage methodCall, ProxyOperationRuntime operation)
   at System.ServiceModel.Channels.ServiceChannelProxy.Invoke(IMessage message)

Exception rethrown at [0]: 
   at Microsoft.Crm.Sandbox.SandboxOrganizationService.Execute(String operation, Byte[] serializedRequest)
   at System.Runtime.Remoting.Messaging.StackBuilderSink._PrivateProcessMessage(IntPtr md, Object[] args, Object server, Int32 methodPtr, Boolean fExecuteInContext, Object[]& outArgs)
   at System.Runtime.Remoting.Messaging.StackBuilderSink.SyncProcessMessage(IMessage msg, Int32 methodPtr, Boolean fExecuteInContext)

Exception rethrown at [1]: 
   at System.Runtime.Remoting.Proxies.RealProxy.HandleReturnMessage(IMessage reqMsg, IMessage retMsg)
   at System.Runtime.Remoting.Proxies.RealProxy.PrivateInvoke(MessageData& msgData, Int32 type)
   at Microsoft.Crm.Sandbox.ISandboxOrganizationService.Execute(String operation, Byte[] serializedRequest)
   at Microsoft.Crm.Sandbox.SandboxOrganizationServiceWrapper.ExecuteInternal(OrganizationRequest request)
   at ExportaDedupeCode.ExportaDedupeCodeClass.Execute(IServiceProvider serviceProvider)Detail: <OrganizationServiceFault xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.microsoft.com/xrm/2011/Contracts"><ErrorCode>-2147220891</ErrorCode><ErrorDetails xmlns:d2p1="http://schemas.datacontract.org/2004/07/System.Collections.Generic"><KeyValuePairOfstringanyType><d2p1:key>OperationStatus</d2p1:key><d2p1:value xmlns:d4p1="http://www.w3.org/2001/XMLSchema" i:type="d4p1:int">0</d2p1:value></KeyValuePairOfstringanyType><KeyValuePairOfstringanyType><d2p1:key>SubErrorCode</d2p1:key><d2p1:value xmlns:d4p1="http://www.w3.org/2001/XMLSchema" i:type="d4p1:int">-2146233088</d2p1:value></KeyValuePairOfstringanyType></ErrorDetails><Message>An error occurred in the ExportaDedupeCode.ExportaDedupeCodeClass plug-in:Generic SQL error.

Server stack trace: 
   at System.ServiceModel.Channels.ServiceChannel.HandleReply(ProxyOperationRuntime operation, ProxyRpc&amp; rpc)
   at System.ServiceModel.Channels.ServiceChannel.Call(String action, Boolean oneway, ProxyOperationRuntime operation, Object[] ins, Object[] outs, TimeSpan timeout)
   at System.ServiceModel.Channels.ServiceChannelProxy.InvokeService(IMethodCallMessage methodCall, ProxyOperationRuntime operation)
   at System.ServiceModel.Channels.ServiceChannelProxy.Invoke(IMessage message)

Exception rethrown at [0]: 
   at Microsoft.Crm.Sandbox.SandboxOrganizationService.Execute(String operation, Byte[] serializedRequest)
   at System.Runtime.Remoting.Messaging.StackBuilderSink._PrivateProcessMessage(IntPtr md, Object[] args, Object server, Int32 methodPtr, Boolean fExecuteInContext, Object[]&amp; outArgs)
   at System.Runtime.Remoting.Messaging.StackBuilderSink.SyncProcessMessage(IMessage msg, Int32 methodPtr, Boolean fExecuteInContext)

Exception rethrown at [1]: 
   at System.Runtime.Remoting.Proxies.RealProxy.HandleReturnMessage(IMessage reqMsg, IMessage retMsg)
   at System.Runtime.Remoting.Proxies.RealProxy.PrivateInvoke(MessageData&amp; msgData, Int32 type)
   at Microsoft.Crm.Sandbox.ISandboxOrganizationService.Execute(String operation, Byte[] serializedRequest)
   at Microsoft.Crm.Sandbox.SandboxOrganizationServiceWrapper.ExecuteInternal(OrganizationRequest request)
   at ExportaDedupeCode.ExportaDedupeCodeClass.Execute(IServiceProvider serviceProvider)</Message><Timestamp>2014-01-08T09:41:13.8880523Z</Timestamp><InnerFault i:nil="true" /><TraceText>
[ExportaDedupeCode: ExportaDedupeCode.ExportaDedupeCodeClass]
[6e8fcb4c-b977-e311-8462-6c3be5bedcf4: ExportaDedupeCode.ExportaDedupeCodeClass: Update of contact]

ENTER: ExportaDedupeCode.ExportaDedupeCodeClass.Execute
0
update
contact
depth: 2
update code? True
build code
update contact

</TraceText></OrganizationServiceFault>