This was reported previously and the following reply was posted by the moderators.
Folder naming convention – You have told us that the new email platform does not allow the use of certain special character and this is causing some problems.
Our recommendation on using special characters in the naming of folders is to avoid special characters in folder names if possible or stick to characters within US-ASCII character set to reduce the risk of problems. # is useable but not currently at the start of a folder name.
Do you access your email using an email client/app on a mobile device. If so try changing the folder's name from within the email client/app. This should then synchronise with the BTMail servers and change it on webmail when you log on via browser.
If that doesn't work call BT 0800.800.150 and and asking for the email team and report the problem to them. They should be able to remove the illegal characters from your folders names.