PE is right about backs. EI backs with gray dark slide plastic handle and double release wont allow you to remove dark slide when back is not attached. Not sure if thats related to back or to Etrsi but also if you have shutter not cocked but you swapped back and back was already advanced - back wont advance film if you will cock the shutter = you wont loose exposure.

Shutter problem - your concern are those indicator lights not working or the shutter itself?