Executing this part of method I got exception in client, exception is:

Cannot roll back s0. No transaction or savepoint of that name was found.

Examining the debugger I have found:

using (VersionValidator.Attach(versionSet))
{
    using (var transaction = Transaction.Open(TransactionOpenMode.New))
    {
        var item = Query.Single<T>(id);
        using (var ir = Validation.Disable())
        {
            foreach (var keyValuePair in CleanUpValues(values))
            {
                item[keyValuePair.Key] = keyValuePair.Value;
            }

            ir.Complete();
        }

        Validation.Enforce();

        ...
        ValidateComments(item);
        ...

        transaction.Complete();
    }
}

The first exception occurs in

item[keyValuePair.Key] = keyValuePair.Value;

System.Reflection.TargetInvocationException was unhandled by user code
  Message=Exception has been thrown by the target of an invocation.
  InnerException: Xtensive.Storage.VersionConflictException
       Message=Version of entity with key 'GridSetting, (dc60186a-8f74-442a-a2b7-f363d5de0eef)' differs from the expected one.

Later this debugger jumps to Disposing of transaction scope and there is my exception: Cannot roll back s0. No transaction or savepoint of that name was found.

I was trying to reproduce this in isolated project, but without any success.

One more question is why this version mistmach occurs: ValidateComments may throw exception, and as far as I understand transaction should be rolled back and version should not change, but it has changed.

Could you indicate possible causes of this problem to create this in isolated project?