I have a new iPad and removed and reinstalled Joplin. Latest Joplin and latest iPadOS.
I do not use encryption.
I set up sync with Dropbox, as I have on my Mac and iPhone, but it gives the error 'no such master key'. See attached image. I removed and reinstalled Joplin again, same error.
How can I fix this so my notes become available on iPad too? I don't want to wipe everything (thousands of notes).
Maybe you can download the Joplin folder from Dropbox to your PC, delete the activeMasterKeyId, set the sync target to the downloaded folder then test if it works.
My idea is that you can test if deleting activeMasterKeyId works on your local PC. By doing this you won't modify anything on your Dropbox so your data should be safe, and you can do whatever you want to the local backup.
To do this, you need to make a copy of the Joplin folder on your PC, then switch the sync target from Dropbox to filesystem.
So, last night, on my iPad, in Dropbox, I removed info.json, started the Joplin app and nothing happened. Really nothing. So I quit the app again, went to the 'deleted files' section in Dropbox and restored info.json, started Joplin: same error. Then I edited info.json from within Dropbox, and removed the activeMasterKeyId part, saved it and started Joplin. Now the app started with Fetched items: 1344/1500 and stayed that way for 5 minutes. I stopped the app, started it again, and now it started with Fetched items: 344/500 - looked familiar, only the 1 was now missing. Now I don't remember what I did, but suddenly there was this orange message at the top that the sync target needed upgrading. I tapped the orange bar, and it started syncing. This was at 01:00h, now it's 09:11h and fetched 35995 items and created 24750 local items.
My iPhone still syncs normally, my Mac app says it needs to upgrade the sync target, and after doing so, it also syncs normally.