One folder on one Android phone will not sync

[PacketMan]

Its been a while since I have posted because Resilio has been working so well for me. So, we just got two identical Android phones; one is about two weeks older than the other.  Both are LG Xpower3 phones.

A bit of background first:

• I have a few FreeBSD servers all running full sync for multiple folders we will call A to G. Rock solid.
• Also have one Ubuntu laptop running full sync to two of those folders A and B. Also rock solid.
• Two weeks ago I added Sync to a brand shiny new LG Xpower3 phone, and added A and B, but in "Selective sync"mode.  Also rock solid.
• Yesterday we got a second brand new LG Xpower3 phone. I can add A in "selective sync" with no issues, and it seems to be perfectly fine.
• When I add B on this second phone, after a couple minutes the app stops responding and you can feel the back of the phone heat up. The syncing process says it synced, but the files in there are greyed out, and even if I touch them to sync nothing happens. If I force stop the app and restart I get the same result.

I have tried everything I can think of including removing and re-adding B, clearing cache, uninstalling and reinstalling (and I did that also trying to deleted those two sync folders created in Downloads to purge any hidden data).  Nothing I try matters.  A works fine, B just locks up the app and heats up the phone.  If I add A and nothing else, Sync seems fine for as long as I let it remain. If I add B by itself the app freezes.  Note I even currently have no files in B, I removed them all just to see, and I let all the other devices sit for 30 minutes (it was supper time) before I tried adding B on this second phone.

The only other thing I noticed is when I touch the sync folder inside sync it takes a long long time to show the contents, whether it be nothing or files.

I assume all phone versions are correct, but have not checked yet. And all machines, except one FreeBSD server, sits on the same LAN (L2 broadcast domain). And my two weeks old LG phone works perfectly fine on wifi networks outside my home network, as well as the mobile data network.  I'm stumped, any thoughts? 

Edited August 24, 2019 by PacketMan
Fixed a typo

[PacketMan]

Well looks like I got it working. Only through trying everything, I decided (for no other reason but to try something) to stop sync running on my Ubuntu machine.  As soon as I did that the LG phone immediately started downloading, and was synced shortly afterwards.  I was quite surprise actually. I waited a couple hours, and have since started sync back on the Ubuntu machine. So far it seems all machines are staying in sync. Time will tell.

I'm guessing this behaviour has been seen before, but I'm not going to search through years of discussion posts. If anyone does indeed know that this is an issue that seems to be still around then let me know please. If this only surfaces again, when I add a new device I think I can live with that.