I use the Gulden desktop app (v2.3.0.4) on MacOS 11.1 in which I keep my Guldens on a Restored wallet (from a passphrase). I send and receive Guldens on that account.

I also have some Guldens on a paper wallet which I'm trying to add to my main account balance.
I found a way to import it (Tools > Import Key > Private Key) and it show's up as an Imported Key account including the balance. But I can't seem to send the balance to my main account.

When I give the order to send the full balance to my main account, it gives a warning that the transaction fee will be subtracted from the amount. When I press Okay, nothing changes and every time I press send, this message pops up.
When I subtract a small amount myself and press Send, a warning shows stating this type of account only support emptying the entire balance in one go. It seems I'm stuck.

How do I send the balance from this paper wallet (private key) to my main account (or to any address for that matter)?

Thanks for any support.