Interesting issue (to me), had a folder under Inbox in my Office 365 account that was called +Cabinet (Receipts/Invoices). While my Calssic Outlook client handled this without an issue, the eM Client did not synch it. I changed the name to just Cabinet and voila it is now synching. Interesting or just FYI - you decide.
I’d say you were lucky Outlook accepted a plus sign in a mail folder name.
Plus signs are not normally used in folder names due to they can eg: cause compatibility issues across different operating systems and applications.
Also they are sometimes reserved for specific functions like eg: mail keyboard shortcuts etc.