Unable to create mailbox error
I used to have a few custom mailboxes, with Coupons being my favorite. However because for some unknown reason Sent messages were not accessible on my iPhone since February (though available through my ISP webmail), I deleted the email account on my iPhone and recreated it.
Now I can get all incoming mail, and I can see those in Trash when I delete them, these two mailboxes are the ONLY mailboxes on my iPhone, while the custom mailboxes are still available via my ISP webmail. Following the instructions in https://support.apple.com/en-us/HT207213, every attempt to create Coupons or any other custom mailbox fails with an "Unable to create mailbox" error.
Searching the community, I see an old post from 2018, but nothing current. Can anyone help?
iPhone X