When I encounter that kind of a problem, I just submit the correction in several parts and it works very well.

I think that it's even possible to copy the correction if it has any kind of formatting, so there is no need to do it all over again.   

Thanks Dragan,

Yes. That's how I got around the problem too.

Thanks Phil,

I resubmitted successfully without any or the formatting - strikethrough and highlighting - and this worked so you may be right.

The "Characters remaining" count was still showing around 1,600 though so I don't know.