"cURL error 60: SSL certificate problem: unable to get local issuer certificate"

Hi,
I’m getting this error for a while now and tried to solve the problem by following the many, many similar posting in this forum (and elsewhere…) - no success so far
Log:
{“reqId”:“FHdtagm+JY9B6sJDeRub”,“remoteAddr”:"---------",“app”:“core”,“message”:“cURL error 60: SSL certificate problem: unable to get local issuer certificate”,“level”:3,“time”:“October 16, 2016 07:11:13”,“method”:“GET”,“url”:"/owncloud/ocs/v2.php/apps/notifications/api/v1/notifications?format=json",“user”:“xy”}
{“reqId”:“TLh9oP5RfBbWq3Gg8bPY”,“remoteAddr”:"---------",“app”:“core”,“message”:“cURL error 60: SSL certificate problem: unable to get local issuer certificate”,“level”:3,“time”:“October 16, 2016 07:11:20”,“method”:“PROPFIND”,“url”:"/owncloud/remote.php/webdav/",“user”:“xy”}
{“reqId”:“xdNjAXffUwJL19iA0ta1”,“remoteAddr”:"---------",“app”:“core”,“message”:“cURL error 60: SSL certificate problem: unable to get local issuer certificate”,“level”:3,“time”:“October 16, 2016 07:11:43”,“method”:“GET”,“url”:"/owncloud/ocs/v2.php/apps/notifications/api/v1/notifications?format=json",“user”:“xy”}
{“reqId”:“Anc6B8DSnv0HbvxsJG4G”,“remoteAddr”:"---------",“app”:“core”,“message”:“cURL error 60: SSL certificate problem: unable to get local issuer certificate”,“level”:3,“time”:“October 16, 2016 07:11:50”,“method”:“PROPFIND”,“url”:"/owncloud/remote.php/webdav/",“user”:“xy”}

I’m running two instances of nextcloud 10.0.1 and using selfsigned certificates on both.
To use the federated cloud feature I imported the selfsigned ca certifcates on the appropriate other instance.
The sharing via federated cloud instances didn’t work but the logs on one(!) now where filling up with these errors (4-5 errors/minute)
Because I didn’t make a systematic analysis when I started experimenting (no time…) I’m not sure if the errors are caused by the sharing.
I removed all federated shares and also all ca certs but it changed nothing.
Is there any possibility to mute curl?
Or maybe get it working as intended?

PHP-version on the instance without errors:
PHP 5.6.26 (cli)
Copyright © 1997-2016 The PHP Group
Zend Engine v2.6.0, Copyright © 1998-2016 Zend Technologies

PHP-version on the instance with errors:
PHP 5.5.14 (cli)
Copyright © 1997-2014 The PHP Group
Zend Engine v2.5.0, Copyright © 1998-2014 Zend Technologies

Thanks in advance
Jürgen

Which OS and which curl version are you using?

Sorry for the late (and incomplete) answer. I’m using openSUSE tumbleweed.
I cannot tell you the curl version as I’m not @home in the moment. But the
system is updated weekly, so I think it’s a recent version.