Hi Jodamo,
To help us troubleshoot your problem, which version of Excel are you running?
There's a couple things that might be interfering with your Excel cell dragging function.
If you’ve been using Remote Desktop, you may get the cannot copy to clipboard error because Remote Desktop uses the clipboard and it can cause weird issues with Excel.
Are you using Remote Desktop? If so, use Ctrl + Alt + Del to open your process manager and kill the rdpclip.exe process. Check any other type of remote desktop applications, VNC applications, download managers, etc. - or any application that might be "monitoring" your clipboard. Disable those and see if you still experience the problem.
In other words - try rebooting your PC, killing unnecessary background processes, and then running only Excel and see if the problem recurs. If it does, the error is happening within Excel and is unrelated to third party software.
In that case, try, within Excel, browsing to Tools > Options > Calculation and selecting "Automatic."
If you're running Excel 2003 or earlier, you can also try browsing to Edit > Office Clipboard > "Clear All" to clear the Clipboard. Also, select "Options" and choose "Collect without showing office clipboard."
Please let us know if any of these suggestions helped!