From reports by others, it appears highly likely that it is about the way you formatted the card. There are two ways.
You probably formatted the card to be part of the common internal storage (“phone storage”), allowing to move apps to the card. However, for quite a while now this way of setting up the card is broken and constantly causes saving errors (and/or the loss of photos meant to be stored on the card).
For the time being (at least until this is finally fixed in a future update or rather: upgrade), it can only be recommended to re-format the card as portable/external storage. You won’t be able to move/install full apps onto the card this way, but data should be able to be moved to the card easily (although not always quickly). You might still find ways to save photos directly to the card upon shooting them, but if this isn’t possible, you can still move them manually.
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