Starting with hugo v0.153.0 and (reproduced on hugo v0.153.2+extended+withdeploy linux/amd64 BuildDate=unknown VendorInfo=docker), the following content adapter fails to build with the following error:

error building site: assemble: failed to create page from pageMetaSource /datenbank/_content.gotmpl: "…/content/datenbank/_content.gotmpl:1:1": unmarshal failed: invalid character '{' looking for beginning of object key string

It still yields the same error message with the following one line adapter:

{{ $dbBaseURL := "foo" }}

The referenced adapter was working for years and stopped working with v0.153. After downgrading to v0.152.* the error disappears and the page builds again.