I had the same problem for like, a week and this was so f*cking unnerving. Nintendo I love you but error codes exists for a reason.
So, I'm not
entirely sure what unlocked the situation but here's the summary:
My switch(OFW) transferd island data via the eshop tool to friend's(CFW) switch.
I messed with the save file with NHSE (WITHOUT launching the game on the CFW first).
I launched it on Ryujinx emulator though.
I tried to send it back to my switch with OFW (many times).
They recognize each other and begin the transfer. Then, boom, communication error.
I tried reboot, hard power off, cutting the wifi/shutting out router, creating a new islander, deleting it, changing things with BCAT (as you mentionned you did it
@dmdsoftware ), trying to send it to another OWF switch (did'nt work, so it seems my switch wasn't the problem, so maybe it's different from op problem) and I was about to try transfering to a switch lite.
And finally, deleting a file/folder named "MovingMark".
Yes, "file/folder" because it had been a file (with no extension) some times and a folder other times (???)
Here's my hypothesis:
This file/folder has to be non-existent when you transfer your island back. It should be deleted by the game when you lauch it, but Somehow I still had it the the tool didn't want to transfer?
I won't test this further as this took me too much time already, but if you have a similar problem, (do backups) and try deleting it on your dump.
I hope this will help, have a nice day!