VC2001: Block name isn't a valid custom element name

Block and page layout names are used as element names, so they must be valid custom element names: lowercase, starting with a letter, with at least one hyphen (hero-banner, not Hero or banner).

How to fix

Create a block with a valid name and use it in place of the old one. Existing blocks keep working; the name only has to be valid when a block is created.