Posting this in case it helps anyone with sync issues. This is NOT a Joplin problem.
I was having sync problems and it was due to Dropbox not letting Joplin through as I was running a VPN. As soon as I turned off the VPN, the sync worked.
When I tried to log in to Dropbox with the VPN on, I had to jump through multiple captchas and never was able to get it. When I bypassed the VPN, I got right in. I believe this shows that Dropbox doesn't like people using VPNs. Note, this didn't seam to be the case in the past.
I now only run Joplin without going through a VPN.
I would be dropping the Dropbox rather than the VPN. Dropbox is probably required to keep track of IP addresses just as they have to monitor what people are storing on their platform. If they detect illegal images they need to have an IP to direct the authorities to. A VPN means they may not have a real address.
I hear you. I wanted to use Joplin with pCloud, but I never got that working. So to save some money I decided to use Joplin with Dropbox as I already had some free storage there. I am now seriously considering just getting the Joplin Cloud service.
What I've found with sites that don't like VPNs is if I tether my phone to my PC using Wi-Fi and use the VPN (Proton) on the phone, the sites I visit using the PC have no idea I'm using a VPN whereas if I use the VPN on the PC they complain or don't work.
For my PC (Linux) I use the split tunnel option on Mullvad to run Joplin and that works fine. The same is available on my phone (Android) but I haven't tried it.
I switched from Dropbox to Joplin Cloud almost 2 years ago. In that time, there was about 1/2 hour where cloud had a glitch and I couldn't sync. I went into these forums, and Laurent said he had an issue and expected it to be resolved soon. It was. Otherwise no issues for nearly 2 years.
I should mention that I'm constantly using an android tablet and a Windows desktop and sync a lot, at least once a day and I'm fanatic about syncing. When I'm done using it on my tablet, I don't just shutdown, I sync and then shutdown. I don't do anything with the Windows computer, but it also doesn't shutdown.