Since the beginning of October Google seems to have kicked me out of their index. At least most pages, including the startpage, are no longer listed. To be honest I have no glue what has happened. I do not see any issues in the Google Webmaster Tools Dashboard. It seems that they are crawling my pages correctly but do not index most of them.
I have now removed some of my WordPress plugins which might have had an influence but I am still unsure if I found the root cause.
So if you are relying on Google to find new content on my pages, I recommend to directly call my page from time to time to make sure that you do not miss anything.
In the meantime I can only wait and hope that Google will add my pages again after some weeks.
Just realized that SSLv3 was active again in my SSL configuration. No idea when that happened. So maybe that was why Google banned my site (which is HTTPS only) from the index. Corrected that now, and I get an A+ grade again from SSLLabs 🙂