I decided to encrypt my notes and added encryption password on iPad.
Soon thereafter I got a FileLock error which appears to be a known nexcloud bug.
Since the nextcloud does not belong to me and I am not an admin, I can not add redis, which is what @laurent suggested.
I decided to delete the encryption - deleted it on iPad, but it still can not sync due to a sync error.
OK. I went to nexcloud and deleted all files. Next opened Joplin on Mac and asked it to force sync all files to the cloud - it gives me an error the the file is locked again?! There is nothing on Nextcloud, what is locked now?! Attempts of enabling the encryption failed as it asks me for a master password - which master password? I've never set up one. Found this thread: Confused about encryption password which appears to recommend to remove the main directory of joplin - does not make any sense - it would delete all my files. It seems that every few days I am struggling to just maintain my sync going. What can I do to fix it?
Before doing anything else you need to change your Nextcloud provider because that one is not reliable, and you can't fix it on Joplin side. The encryption is not an issue.
If you want something free, consider using Dropbox maybe, which is slow but at least it works.
I had a lot of issues with a cheap NextCloud provider with constant file locks that I couldn't remove even after deleting the entire content of the sync target and making it from scratch. I moved to a locally hosted NextCloud server running on an RPi and haven't had a single issue since (even without redis). (Although I plan to create a Joplin Server instance when I have the time and a strong enough desire)
I wonder if it depends on the file system. Every time I tried running a Nextcloud instance with default settings, it would lock within 5 minutes of use, so I'm surprised it doesn't in your case.
Followed your advice, @laurent and switched to Dropbox for a trial. In fact I exported the database in jex format, started completely new joplin database and imported jex data in. Set up Dropbox as a sync target. It synced 1001 notes and got stuck, nothing happening for an hour. Restarted Joplin - now it is stuck on "created remote item 1" for an hour.
Reset the database as above again. The encryption is disabled. No problems syncing. So I tend to think it is likely to do with encryption.
Sorry I inadvertently lied, I just checked and it does have redis memcaching configured, it was just a default option of the package I installed for nextcloud. Either way, a million times better than the hosted solution I was using.
Apologies, I since completely reversed back to the unencrypted notes and there is no way for me to re-create the error log without going back and encrypting notes again. I will plan to do it again when I will have time and energy to go through this process again.