[GraemeJ]

Although not directly the answer to your problem (it seems Mike is on top of that) this sentence caught my eye.  " A day or two ago, I launched MobileSheets to find a pop-up that looked like an explanation/log of fixes implemented in some recent automatic update (?)".

I would advise turning off any automatic updating, particularly for programs that are 'mission critical'.  Any update is a risk (e.g. accidental introduction of a bug - as this one did for you) and it is far safer to leave any updating for when you have the time to check out everything is working.  A flawed auto-update has the potential to ruin a gig!