Very odd, I thought one of those would help.
Can you delete files in the folder or using a PowerShell (Admin) command. I don't know where this folder is but assuming it is in your users folder then the command would be del C:\users\accountname\dropboxfoldername or
rd C:\users\accountname\dropboxfoldername.
Can you provide a screenshot of the folder location in your reply so I can be sure the command line is correct.
You can try the same in Safe Mode or with a clean boot
https://support.microsoft.com/en-gb/help/929135/how-to-perform-a-clean-boot-in-windows
to see if either of those modes releases the folder/files for deletion.