in 9.1.0 (and 9.1.1) adding a custom block results in an error “undefined variable $…” for the first block variable used in the form (shared form for view, edit, add). editing existing blocks works fine. the problem is there for ALL custom block types.
I guess it’s not a problem with Concrete itself, but you probably updated php version to 8+ on your server. Undefined variables (often in custom blocks) are common error that need to be fixed before switching to php8.