Hi OneNice,
I have an identical issue and I can reproduce it on multiple MacBook Pro running the latest Mojave, Catalina and Big Sur. I can consistently reproduce the issue with a CalDigit TS3+ or Belkin USB-C to Gigabit Ethernet Adapter. On Wi-Fi it works although I'm seeing other issues on Wi-Fi with a SMB mount.
I'm engaged with Apple Support but so far they aren't sure.
Ben---
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OneNice,
I tracked this down to something specific to macOS and a Samba bug (Bug 14420). Try setting the following in your smb.conf. I've had no issues since.
fruit:zero_file_id = yes
For anyone still having problems where Finder freezes/hangs on copies and/or 0 byte files are created I tracked this down to a Samba bug (Bug 14420). Try setting the following in your smb.conf. I've had no issues since.
fruit:zero_file_id = yes
If you aren't loading fruit in vfs objects it won't have any effect. The default for that setting used to be yes and it was changed to no which is causing problems. Try explicitly setting "fruit:zerofileid = yes" in your smb.conf. The following is working for me and I was previously seeing the some problems you are. I omitted any values not related to macOS.
[global]
		ea support = yes
		fruit:aapl = yes
		fruit:copyfile = yes
[share]
		fruit:metadata = stream
		fruit:resource = stream
		fruit:zero_file_id = yes
		vfs objects = fruit streams_xattr