Hi Dionysos Welcome to the forum although not a great time your having.
As @yvmuell has considered you may have formatted the card, once you inserted it, as Internal storage rather than Portable, whereas this issue is not specific to Fairphone when an SD card is inserted the top option is to format as Internal.
a) If you choose internal the SD card is encrypted as are the contents of the Phone memory so you will not be able to view the contents once you remove it from the phone
b) if you remove the card the phone may not function properly as being now a part of the internal memory some app data may be stored on it too.-
For more detail see :-
Check the SD card Settings > Storage to the right of the SD card would be an eject icon if you have formatted it as Portable.
See also comments on the link below you are not the only one with similar problems ~ it’s not really a Fairphone thing.