Expanding view categories on web

We have a db with lots of categorised views. Through the notes client these views work as expected but via a browser clicking on one category will expand another. Any ideas why?

The problem occurs in the production environment but I cannot reproduce in dev - could it be due to a larger number of documents in production? Even so, this is something we need to fix - but not be deleting documents!

(In development a slightly different problem occurs - one specific category cannot be expanded - the documents inside the category are not hidden using readers fields or anything, and again, through the notes client they are visible. Production problem is most important tho, I just thought I’d mention the dev problem as I think that they are probably related…)

Many thanks in advance!