If you google this error message the link below shows up that basically says you have to remove special characters from the front of your XML file any maybe check the encoding (UTF-8 vs. UTF-16).
Maybe you can give us a little more context:
what is your operating system
what is your file system
are you on a local machine or a server
do you use any online(-backup) services like one-drive where your workflow is
have you checked that your hard drive is OK
does this error occur with all your workspaces or just one
have you made any recent changes
you could try and set up a new knime-workspace and then try to copy several folders there and see if they can be opened
could you try again and send us the log file (maybe delete or rename the old log before that)