Yesterday everything was alright and today the notes between my Smartphone and laptop are not syncing.
I am using nextcloud.
I think your Nextcloud provider is down.
It's not you, nor Joplin.
With your browser see if you can connect directly to your Nextcloud account.
I use mark.nl.tab.digital, and I sometimes have this error. Usually everything goes back to normal after a few hours.
You are right.
I changed the provider and now it works.