Hi, which SFOS version are you using?
Have you tried to launch the app from the terminal?
I had the same issue sometimes on my XA2. Mainly with Messages, Notes and Calculator.
I never found the cause but found a workaround which is keeping apps open in partnerspace.
This allows you to keep some apps running in background and access them quickly by swiping from the right to the left.
For official Partnerspace docs, see here:
Example for Calculator, Messages and Notes (command as user, not root/devel-su):
dconf write /desktop/lipstick-jolla-home/partnerspace/applications “[‘/usr/share/applications/jolla-calculator.desktop’, ‘/usr/share/applications/jolla-messages.desktop’, ‘/usr/share/applications/jolla-notes.desktop’]”
I’d also recommend using the patch “Blur Partnerspcee Background“, thanks to @nephros, which greatly eases the readability.
To see how to enter commands via ssh (from the PC), also see here.
This said, it would be nice to find the real cause of this issue.