Error writing Metadata when adding nested keywords

For a while now, I have had an issue where when updating keywords, it comes up with an error the first time, and is usually ok when I try it again/

On more serious occasions it removes the metadata (created date/keywords/location data etc.

I use nested keywords, and it tends to happen when either I enter a deep level or do too many updates in quick succession.

The error message is not useful, and I dont know whether I could add some debugging to hget more info. (If I call support it tends to work!)