Have you tried without antivirus? (Including Defender.) It will be involved in all file copies so it could be a factor, especially if it’s only going wrong with some files and not others.
If Preferences / Miscellaneous / Advanced [Filesystem]: copy_nonbufferio_threshold has been modified (the value will be shown in bold if it has), please try resetting it. Same with copy_buffer_size above it.
Process Monitor logs of what happens when the copy fails may also reveal something: Process Monitor instructions