For streaming encrypted channels it kinda depends how long decryption requires, if I switch to encrypted one and open the stream immediately, in some cases if you do it to early, you see black picture only, but if you wait 30-60 seconds, picture should come. Restarting the stream fixes it. I am not really watching encrypted channels but when I did, I used to face it. If your server streams encrypted channels as decrypted, you do not need on your receiver side the cam, only if you send stream encrypted you need it, but that is something you did not describe. Like you were told before, please try it with non encrypted channel first.
After writting that record is black stream I am almost 100% sure that you do not decrypt your streams when saving, there is option in WebIf where you can set it.