Writing to SDE, get error with feature caching off, works with feature caching on!

Hi,

I have spent most of the morning going down a rabbit hole and really am none the wiser. I am writing out some content to a feature class in SDE. When I run the full process with feature caching on or off I get a failure to write with a very unhelpful ESRI error:

An error occurred while attempting to insert a row into the table 'DBO.xxxx' using a cursor. The error number from ArcObjects is: '-2147418113'. The error message from ArcObjects is: {}

However if I run it with feature caching on, up to the last transformer and then run to the writer afterwards the content goes into the SDE, no error.

If I write to a ffs file then push that to sde. It works.

My suspicion is that I am using a feature reader, it seems like SDE doesnt recieve all the right info about the schema before FME tries to write to it, whereas it does when feature caching is enabled?

Any ideas very much appreciated.

Thanks

Oliver