The F-Droid page for OGS (com.ogsdroid) says "The app is still early in development
so be prepared for bugs and half-implemented features." and the latest update there is from August 2017.
Aurora Store (alternative access to free-of-charge Google Play Store Apps) lists “Skolemelding foresatte” (com.cgi.oslomelding.foresatte) as GSF-dependent, which means it needs Google services.
/e/ has no Google services. /e/ uses microG to mimic Google services as good as it can, but that has its limits.
While it works for a lot of Apps, there is no guarantee, and even if it works at a particular time, changes on Google’s side can break that at any given time, until microG did the necessary catching up again.
The App currently isn’t listed as not working here … Editable List of Apps that work or do not work with microG … or here … https://github.com/microg/GmsCore/wiki/Problem-Apps … so your problem with it now might still be something else.
In short: There’s always a possibility stuff just doesn’t work.
Some generic things to try …
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Reboot the phone (Does it work again?)
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Settings - Apps & notifications - (select the App) - Force stop (Does it work again?)
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Settings - Apps & notifications - (select the App) - Storage & cache - Clear cache (Does it work again?)
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What does it say in Settings - About phone - Android version (<- tap on this) - /e/ version?
Did you get a notification that an OS update is available? -
Settings - Apps & notifications - (select the App) - Storage & cache - Clear storage (Warning! This will delete all data the App saved on the phone. Does it work again?)
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Uninstall and reinstall the App (Warning! This will delete all data the App saved on the phone. Does it work again?)