Recently, I was playing Crash Landing on Mine your Mind, and had to redownload the FTB launcher, my friend had been on the island with me. When I got back on the server, I could not go back to my island because he did not have permission to invite anyone. I had started a new island, and want to get back to my old one because I had lots of progress on that one. But I can't because my friend can't invite me and I am stuck on an old Island
If it was just you and your friend on the island and you somehow had been removed from the island without doing any commands and such... I'd say your friend should make a ticket asking for to make him/her the leader of the island that way they are able to add you back onto it (and link this thread in the ticket as it has a lot of additional information). And I say this in assuming that your friend has permission to be on the island still just not have ownership commands.
Sorry to hear that, but as you selected hardcore mode, when you run out of lives the island gets reseted and staff can't do much. Marking this thread as solved. Feel free to open a new one if you have more issues.