Cannot rename Outlook Express folders

On my computer (Outlook Express 5 and Windows 98 SE) I was able to create folders in Outlook Express, move them, rename them and delete them. Now, every time I try to rename the original folders I get an error message: “The folder could not be renamed. An error has occurred.” I can still delete and move the folders. Some new folders I create can have their names changed, even when containing messages but after a few times they also bring up the error message. The problem does not seem to be related to the characters being used in the name change process. Names are not being duplicated.

Does anyone have any ideas?