Thanks William for your kind reply, I will try to get in contact directly with apple and see what we can do together.
Yes, the kernel panic points to com.apple.filesystems.smbfs, but as said I don't always have a kernel panic, only sometimes 1 time out of 50? Most of the time with the following configuration the error is -8084 (and that happens only for some files, always the same, and it doesn't matter the size of the problematic file, because I'm able to transfer other files, bigger or smaller in size, so it's very difficult to understand what is happening in the background).
For these problematic files, the usual behavior is always the same, the transfer seems to start, it stops at exactly 67,1 MB, then it hangs and the system throws -8084 error.
By forcing sync, it fails too, but the transfer hangs with the same error at a multiple of 67,1 MB (if I remember well at 134,2 MB)...very strange..
In Catalina latest stable all is working without issues.
I think I don't have any particular configuration, here is my samba.conf on the linux box:
[global]
access based share enum = yes
bind interfaces only = yes
interfaces = lo enp6s0
[2TB]
path = /media/2TB
browsable = yes
Force User = nobody
valid users = user1, user2
write list = user1, user2
vfs objects =
[6TB]
path = /media/6TB
browsable = yes
Force User = nobody
valid users = user1, user2
write list = user1, user2
vfs objects =
[VM]
path = /media/VM
browsable = yes
Force User = nobody
valid users = user1, user2
write list = user1, user2
vfs objects =