I have added this bug to the tracker:
Issue 2175 – The first copy&paste or drag&drop of remote files to other application might fail in Explorer interface

I'm sending you an email with a development version of WinSCP to the address you have used to register on this forum.

I have sent logs from debug version of WinSCP on email.

I usually run WinSCP without Administrator, but running as Administrator has no effect on this error.

I have sent you an email.

Thanks. Aren't you running WinSCP as Administrator?

Can you send me an email, so I can send you back a debug version of WinSCP to track the problem? Please include a link back to this topic in your email. Also note in this topic that you have sent the email. Thanks.

The video shows these steps.

Attempt 1. Ctrl+C & Ctrl+V created the scp39721 folder. This is always how it works the first time after opening WinSCP.

Attempt 2. Ctrl+V pasted the same folder as if from the clipboard. It always works like this.
Attempt 3. Ctrl+V - just repeating attempt 2 with the same result. Any repetition of Ctrl+V never leads to anything else.

Attempt 4. Ctrl+C & Ctrl+V. This is the second time pressing Ctrl+C after attempt 1. It always results in the file transfer being successful. Moreover, I have seen in some situations that this leads to the message "you want to replace the file", as if pressing Ctrl+C again allowed the previous press to end correctly as well.

Then I closed and opened WinSCP.
Attempt 5. Ctrl+C & Ctrl+V created the scp41821 folder. This is always how it works the first time after opening WinSCP.

On the video I can see that the download worked on the third attempt, right? Is that random? Does it work sometimes and sometimes not? Or did you do something differently on the third attempt? (it does not seem so from the video).

I used folder C:\WinSCP.

Please locate the Enable value in registry as shown below and create new LogFile value along it:

Restart your computer and try the copy-paste. Attach the log file afterwards.
Can you also record a video of what exactly are you doing?

Changing "Append session name to temporary path" does not affect the error.

Please post screenshots of your "Transfer > Drag & drop" and "Storage" preferences.
https://winscp.net/eng/docs/ui_pref_dragdrop
https://winscp.net/eng/docs/ui_pref_storage

NTFS external USB HDD (D:), FAT32 external USB flash drive (E:).

Disk C: is internal SSD. But today, for some reason, after trying different folders, I always got the same error when pasting from the buffer, even to disk C:. At the same time, drag and drop always works correctly.

If I copy from the server (FTP, SFTP) and paste the file on the disk via the clipboard, I get an empty folder. If, after that, I press Paste (Ctrl+V) again, I get a message about the merge folders, as if this folder was already in the clipboard (Image 1).
If I repeat Ctrl+C on the file from the server, and then Ctrl+V in target folder again, I will receive the file (List.xlsx). But when I restart WinSCP, I will be prompted to delete another temporary folder (Image 2).

What kind of drives are the other drives?

When I try to transfer a file or folder by copying to the clipboard in the WinSCP window, and then pasting into the windows explorer folder located on a drive other than C: where WinSCP is installed. Then I get an empty hidden folder scpXXXXX (where X is digit) in the target folder.

If I reopen WinSCP, it suggests deleting a temporary folder with the same name.

There is no error if you use drag and drop or to disk C:

Using WinSCP 5.21.7 with “Explorer” interface on Windows 11 22H2 (and Windows 10 1909).