Yes, Galaxy S10 and Android 10 here, and Wifi Sync recently started spectacularly failing for me too. This was weeks after the Android 10 update. Now MB Wifi Sync acts like it can't access the SD card although permissions are granted and double-checked. Cleared data for the app to no effect, uninstalled the app rebooted the phone and reinstalled to no effect. Deleted the device profile and recreated it to no effect.
Buttons are not even clickable in the app for me. I can press them all day and nothing happens. Only when I change to "Internal Storage," run a preview, and then go back to "SD Card" can I click anything. But then I get the errors like it has no access to the SD card.
This started mid-sync for me. It partially sync'd, then the errors started and now I can't do anything. I don't know if it's an Android issue or an issue with an overnight Windows update or what, but it worked one day and then not the next.
Back to wired sync for me. It takes 45 minutes to scan, but at least it works.
just checked again after I read your post.
Cleared app storage, so I could start from a "blank slate".
Selected "internal storage" from the start (even though I have hardly any space left on internal and want to keep my music on SD).
Worked like a charm. Didn't even do a wired sync or selected internal storage in MB on my PC, Just changed my sync settings from a configured device to include only the single playlist, did a preview and synced it to the internal storage with no problems
Cleared app storage again, started anew, selected "removable SD card", and preview/sync buttons stopped working again.
repeated steps to sync internal storage, worked again.
Tried back and forth about five times. Now going to see if it still works after rebooting PC and Phone
I'm willing to place a small bet there is some sort of issue with SD card acces in Android 10.
Edit: Confirm it still works like described above after rebooting several times and setting up the app. also reinstalling has the same effect as clearing app storage.
Reverse sync doesn't work due to the GMMP unlocked version error (even though I have it unlocked) but that seems like something else entirely so don't care bout that now at all.
I would like to carefully draw two conclusions;
1. I think the issue exists on the phone side of syncing. Nothing to do with network or MB on the desktop.
2. The issue only seems to manifest when syncing to SD card for me
I can vaguely find something about storage acces being a little bit different in Android 10, so that might also be something to look at, although I have several other old apps, some not updated for over a year, that can read/write to SD card just fine.