NextCloud:FTPS证书验证

I try to connect with an FTPS-Server via my NextCloud-Instance. Unfortunately this is not possible, I just see the following error-messages in the logs:

Error PHP stat(): stat failed for ftps://xxx:xxx@ftps.domain.me/ at /var/www/html/nextcloud/apps/files_external/lib/Lib/Storage/StreamWrapper.php#127 2017-03-17T12:59:42+0100

Error PHP stat(): SSL operation failed with code 1. OpenSSL Error messages: error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed at /var/www/html/nextcloud/apps/files_external/lib/Lib/Storage/StreamWrapper.php#127 2017-03-17T12:59:42+0100

Any ideas?

For my NextCloud-Instance I use a SSL-Cert from Let's Encrypt, the FTPS-Server uses an StartCom (StartSSL) Free-SSL-Cert...

Thnx! :-)

Cheers!