A couple of things
a) you could start in safe mode. This will disable any apps you have installed and delete their settings, so the apps will be there when you start ‘normally’ but some settings may have to be tweaked.
b) you could do a factory reset which will not only remove any custom apps but also delete any media/docs etc that you have not stored on an SD card that was formatted as portable.
Ensure you have the latest update as if you cannot find a solution via help here, you may want to contact official support ~at~ fairphone . . . . com
Official page for Safe Mode ~ FP3