Letsencrypt using cloudflare

I installed the letsenrypt service from the catalog and did all the nessecary configuration. on startup, I get the following errormessage in the service:

Error obtaining certificate: Error presenting token: invalid character ‘<’ looking for beginning of value"

I´ve entered my domain, that is registered and active in cloudflare and additionaly a subdomain and I get the errormessge for both domain names.