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When a menu item to select/open/save a file is disabled (dimmed) or when you get the message Storage access framework not available, the storage access framework, a standard Android component, is probably not present. This might be because your custom ROM does not include it or because it was actively removed (debloated). Note that this will result in similar problems in other apps too.
FairEmail does not request storage permissions, so this framework is required to select files and folders. No app, except maybe file managers, targeting Android 4.4 KitKat or later should ask for storage permissions because it would allow access to all files. Moreover, recent Android versions disallow access to all files for apps, except, under specific conditions, for file managers.
-To resolve this problem, the system component Google Play Services may need to be updated. Please see here on how.
+To resolve this problem, the system component Google Play Services may need to be updated. Please see here on how. Please note that you will need to restart your device after the update!
The storage access framework is provided by the package com.android.documentsui, which is visible as Files app on some Android versions (notably OxygenOS).
You can enable the storage access framework (again) with this adb command:
pm install -k --user 0 com.android.documentsui