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DOPUS + Cell Phone

HTC Android + USB Connect, Win10 Explorer opens the Cell phone dir and content as expected.
DOPUS will show folders but no content.
Explorer and DOPUS cannot access the cell phone at the same time.
IF Explorer is used FIRST to open the folders then closed, DOPUS can be launched and is finaly able to see the folder contents.

MTP can be fairly limited, in terms of how many things can talk to the device at once, which may be partially down to the phone end of things.

On Android, I'd usually recommend using FTP instead, as it is a lot more reliable and also works wirelessly:

never considered that, thanks for the idea! I'll do some research.

I've on-off used DOpus' Synchronise tool to sync music to my various Android phones and it's never gone well because of what I'm guessing is the same problem. My experience is that it takes forever for the PC to navigate through the folders on the phone and when I actually start transferring files, it always either seems to lose the connection (no data transfers) or complains that the copy command is the wrong command to be using (I paraphrase, can't remember exactly what it says). Both of these problems can happen immediately or at random during the transfer, and no matter how many files I have, it takes multiple attempts before they're all sent across. I've always assumed that these problems were not caused by DOpus.

I've tried several other options in the past, like scheduled backup tasks and local cloud transfers using various personal cloud options on my PC and/or NAS and none have been particularly good. Right now I'm trying Resilio Sync (again) because the last tool I tried, NextCloud, only allows you to sync to a folder within its own directory, rather than any existing folder, which was stupid. RS lets you choose a different directory (if you enable Advanced mode in the settings), but when I've tried it in the past, it's never really worked well. Fingers crossed.

Synchronise works with FTP so I would advise just using that. MTP is pretty unreliable.

If you are willing to test more: I have had good results with open source solution Syncthing, both PC-PC and PC-Android.

I was mainly looking for something that would work hands-off via my server. My music's automatically synced to the server using NextCloud, but as mentioned above NextCloud couldn't do what I wanted. Never thought of trying FTP to the phone (how does that work, anyway?) but I'd prefer something that would work in the background instead of me having to manually perform the sync each time. The other necessity is that I can run a version of it on FreeNAS, hence trying Resilio Sync this time.

The common problem on the Android end seems to be the requirement for the app to keep a notification icon in the status bar, which I would rather they didn't, but it seems that without it, Android just kills the app and nothing syncs at all. RS Seems to claim that it can work without the icon (prefs have separate switches for notifications and the icon) but given that my phone doesn't appear to have tried to sync in nearly a week, it seems unlikely that the app remained running.