Hi Nedda,
I agree that renaming a file is safer than editing the registry since you can just change the name back or you can even make a copy of the file/folder first. I did not get this error, but I did not rename any folders and I did not rename any files that had a folder with the same name. None of my signatures had a folder and Outlook created the folders for each signature after renaming the files and getting them back in Outlook.
What you should do next can be tricky depending on your level of Windows experience. There are different approaches to backing up your signatures (e.g., create a system restore point, make a copy of the Signature folder with a copy/paste, make a copy of the signature files/folders you're renaming using copy/paste) before you continue with renaming. It could also be that you do not need to rename your signature files that have a matching folder (do these show up in Outlook already?). I haven't had a chance to research what is happening with the Outlook update and the new Outlook preview for a good solid solution.
If you do nothing to try to fix this issue:
If your signatures are missing after an Outlook update, worst case is you go to the Insert tab > Signature > and select the signature to use. If your signatures are missing after upgrading to the new Outlook, I haven't been able to figure out how to get the missing signatures back in Outlook.
I have not tested the following and I cannot confirm that this will work. I cannot not confirm you won't lose all of your signatures if something goes wrong. Do the following at your risk. If the matching folders for your signature files do not contain "(YourEmailAddress)" in the folder name, you could try renaming the folders so they match the updated file names with the added email address. Presumably you could just rename the files and folders back or replace them with the backed up copies if you did that.
I wish I had better information for you. Good luck.