SOLVED 
this was the door to the solution:
the comment by schoen:
(1) It’s possible that the problem is elsewhere in your Apache configuration (in terms of a separate misconfigured virtual host listening on port 443). It’s also possible that there’s some information in your Apache logs either at startup or at the time of an inbound connection that might help in diagnosing this.
here, in this thread:
my problem was that I didn't have error warnings
apache2 restart worked good
certbot renew --dry-run also worked good
nevertheless, I had some certbot renew errors...
just 1 or 2...
of which one was obviously a missing document root directory...
which I created...
anyway the core problem was
that I have 2 IP addresses...
and one vhost was configured at the nameservers using IP #2 (A record)
but in the vhost config it was pointed at IP #2
I changed the nameserver "A record"
and disabled a couple of vhosts...
and all of a sudden lots of errors came up!!!
apache2/error.log showed " Configuration Failed" messages...
and certbot renew --dry-run also showed ALL RED LINES.. full of errors...
and from these
I could figure what else was wrong beside one vhost being pointed to IP #2
whereas the A record at the nameserver pointed to IP #1
one other problem was...
a vhost config error...
I dropped the domain (a year ago!) "the-book.eu" ...
and started using "this-book.eu" instead...
and I changed the vhost file.. manually... not with a global "replace all" method...
and the server name remained the old one...
(a server alias with "this-book.eu" was added, probably by certbot, as a matter of fact, and the domain was usable, as far as I remember, but it wasn't really in use, cause my book is still in the making 
)
after I've corrected this,
all the red lines and errors were gone... (note: red lines during certbot renew were the same at each vhost/nameserver)
and the sites (which I haven't disabled by a2dissite) came back and work like normal...
THANK YOU VERY MUCH for your effort to help with this case...
I hope that the solution will be useful for someone in the future...
once again what schoen said:
(1) It’s possible that the problem is elsewhere in your Apache configuration (in terms of a separate misconfigured virtual host listening on port 443). It’s also possible that there’s some information in your Apache logs either at startup or at the time of an inbound connection that might help in diagnosing this.