hugo -v server fails with bare keys cannot contain '+'

[rei-vilo]

Launching hugo -v server fails with the following error:

Error: Error building site: "/Users//GitHub/Energia/energia.nu/content/_index.adoc:1:1": unmarshal failed: Near line 0 (last key parsed ''): bare keys cannot contain '+'

Mentioned file is _index.adoc

• How to get it right?

Configuration

• Hugo Static Site Generator v0.68.3/extended darwin/amd64
• Asciidoctor 2.0.10 [https://asciidoctor.org]