Not quite sure how to explain this, but I will go through what I did, and when the errors started happening.
I have enabled the default taxonomy view in Views, changed it to fields, and added a grouped filter.
After saving, I see the error
"Warning: htmlspecialchars() expects parameter 1 to be string, array given in check_plain() (line 1583 .../html/includes/bootstrap.inc)." AND suddenly the admin menu has loads of new items in it?
I read up on it, but cant see any fixes, so try disabling and re-enabling the taxonomy view, and also admin menu.
Now the error message has gone, everything works fine, but I am left with all of these extra menu items in the admin menu.
I have logged in as other lower level users, and their admin menus are fine, yet as the super user (with all permissions) I have about 20 or 30 menu items added to the menu before the usual entries.
I have no idea where to start looking with this. The error above seems to happen when a module passes an array through check_plain(), so it usually a module error rather than core. But in this case, it seems to be the taxonomy view that has triggered it, unless its a coincidence.
Can anyone offer guidance?
EDIT: Also tried flushing cache. And worst comes to worse can revert to a backup 5 days ago, but will lose a fair bit of content, so want to avoid this.
EDIT: Looking phpmyadmin, I can see that all of these new menu items have indeed been added to the menu_links table, but do not appear in the structure > menus > management list.