@Sparrow due to some configuration error (that occured since 2 or 3 days ago only) I couldn’t even install update #17 yet, so that’s won’t be the reason (I’m running on update #14). Thanks anyway!
@Fsantiago1979 I installed with “letsencrypt-auto certonly -a manual” , so I’m quite sure there was no interference of any kind with Plesk. The only thing I did was extending the existing httpd.conf files of the respective domains by the SSL sections and adding the LE certificate paths in there.