For months now, my 3DS has had this problem and I've been looking around the internet for a solution. Finally, I found this topic and I've kept tabs on it to see if one would come up.
2. It can be fixed by deleting ALL save data
This is what solved my problem and I was finally able to send out a replay for the first time in weeks.
As a just in case, my card is the
Ultra PLUS 32 GB SD and I have 18 registered friends on my 3DS. I erased
all my old replays to see if it would change anything (it didn't) and after that, I decided to try erasing the data for the game, which then it worked perfectly fine.
I wonder if removing the SD card to upload pictures to a computer/laptop might unintentionally cause the error? Sometimes I would (before Miiverse got the share function update) remove the card to transfer them over and while I can't say with 100% certainty, my replay troubles started around that same time? They're probably not related, but it's something, right?