Cannot save preferences

Hi! I have used Wallch since 2016 with no problem. I only ask it to randomly select from 9 of my own images & change every 15 minutes. Recently (since upgrade to 18.04?) it wants to change images every 10 seconds. When I open Preferences, then the 10 second intervel is preselected. I select 15 minutes & Save but this has no effect & if I open Preferences again it says 10 seconds again! If I go to "Show" then the interval is already set at my old 15 minutes & from that point on, it changes OK every 15 minutes - but only until the next boot, when it reverts to 10 seconds... I have tried removing & reinstalling Wallch with SynApt & also Completely removing & reinstalling, but no good. Thanks for any help!