Also I see from the error message you are using Tab Digital. Do you happen to be using the free plan?
Free nextcloud providers tend to be unreliable from what I have heard, but also if you are not in control of the nextcloud installation, providers can sometimes (though rare, and probably less likely to happen if you pay) introduce bugs which break the integration with software like Joplin.
Here is a recent example where Tab Digital introduced a bug that encrypted user data when encryption was not enabled: