I had a stash category tab in position 0 which I couldn’t delete or rename. If I right-clicked on the tab then it brought up the configure tab screen for the tab next to it. I exited to character selection and it started working again.
Then I deleted the category so I only have one category now. Except my stash tabs no longer show up within that category. I thought I had lost all my gear but I managed to access the tabs through the quick view / tree thing on the right. Again, exiting to character selection fixed it.