Setup is 8.7 SP6 server. Most clients are SP5 or 6.
Preferred servers are setup at each of our 50 sites usign robocopy to keep them synchronised.
I've encountered the first problem across multimple sites.
The client only gets every second file from the preferred server, the other time it comes from the core. In some cases it will get 20 files from the preferred server then alternate getting files from core and prefeerver. I've attached the SDCLIENT.log and i've changed the name of my core and the preferred server to CORESERVER and PREFERREDSERVER.
the preferredservers.dat file references an incorrect server, I saw this today where it referenced a preferred server for another site, what I think happened was a laptop was in the Carlisle office yesterday, received a distribution task so had the carlisle preferred server in the preferredservers.dat, then the laptop went to the Colwyn Bay office, a peer received a push task and looked to the laptop for a cached preferredservers.dat and got the laptop's dat file that pointed to the Carlisle server, then most of the devices on that subnet got a preferredservers.dat that was pointing to a wrong server - should that happen?
You'll probably want to talk to support here, to be honest.
Less because I think there's a defect in here, but more because of the debugging involved. You won't find the exact source (and cause) for "what file I got from where" in the normal logs, this requires debug-logging, and the reading thereof.
It'll be a fair bit better I think to get in touch with support, and to set up a controlled example, so we can see what's going on. The reasons could be plenty from hash-mismatch to whatnot ... we'd need to see what the debug log states in detail.