First of all, I am really sorry you are facing this issue. This is most likely not your fault. First let me explain why this occurred:
You accidentally (or intentionally) installed the v3 beta on one of your devices.
This triggered a migration of all your data.
While on your other devices, you remained on the latest v2 version.
Now every time you sync, it tries to get the v3 changes but it cannot understand them so it gives you the above error.
You are most likely to face this issue if you use the F-Droid version of Notesnook which accidentally received the v3 beta update. We have fixed and stopped any further accidental deployments.
The good news is this is totally fixable:
You must install the latest v3 beta on all your devices (read instructions below on how to do that).
Or you can wait until v3 enters stable (in a couple of days). However, your sync won't work until then.
If you are not getting the above error, there's no reason for you to install the v3 beta. However, if you'd like to help us test things please follow the instructions below.
If you are using F-Droid, you should already be on v3 but it's not the latest version. You can get the latest APKs from GitHub releases
If you'd like to use Google Play Store, follow these instructions:
Join the Notesnook Beta Google Group: https://groups.google.com/g/notesnook-beta
You can choose to not associate the group account with your gmail account
Go to the Notesnook's Google Play Store listing
Make sure you have the same Google account selected as the one you used to join the Google Group
Scroll down in the listing until you see "Join the Beta Program" section
Click on "Join beta"
If it gives you an error like "Can't join. Beta program has ended." close the Play Store and try again (it'll work after a few tries)
Once you are successfully joined, you can install the Beta (you might have to uninstall the non-beta version first so make sure to take a backup).
To use the v3 beta on iOS, you have to install the Apple Testflight app and join Notesnook Beta via this link: https://testflight.apple.com/join/tYrz0XXl
Once you have joined, you will see the latest v3 beta become available in the Testflight app automatically.
You can grab the binaries for your respective platform below:
If you'd like to use Flatpak that's also available:
# Add the flathub beta repository
flatpak remote-add --if-not-exists flathub-beta https://flathub.org/beta-repo/flathub-beta.flatpakrepo
# Install the app
flatpak install flathub-beta com.notesnook.Notesnook
# Launch
flatpak run --branch=beta com.notesnook.Notesnook
You can use the v3 beta via the web app available at https://v3.notesnook.com/