Just one other update on this, in case it's useful for other users...
It seems to be this plug-in that creates an issue on lavender: [nodebb-plugin-category-join-group]
-I had Persona theme - all fine
-I changed to Lavender - all fine
-I installed and then uninstalled above plug-in, and thought nothing of it.
-Problem arose with categories...
-Reverted back to Persona and all was fixed
-Once again reverted back to Lavender, and still everything was ok.
-Tried again to reinstall above plug-in - problem came back again
Again, I have flip-flopped between themes and the problem is sorted, but it definitely seems that the above plug-in doesnt agree with Lavender theme for some reason. Which is a shame, because I'd love to use this plug-in.
I hope this might be useful for others.