Association between two domains

[Gantenbein]

Thank you for your patient and well informed support, MikeMcQ!
It looks like I'm really stuck now.
I tried this and that, and now my installation seems completely messed up...

If you use a tool like this: SSL Checker

What does it show for the SANs names in General Info section? Are these two domains combined?

For a.x, it now shows:

139888418518336:error:1408F10B:SSL routines:ssl3_get_record:wrong version number:../ssl/record/ssl3_record.c:331:
CONNECTED(00000003)

no peer certificate available

No client certificate CA names sent

SSL handshake has read 5 bytes and written 310 bytes
Verification: OK

New, (NONE), Cipher is (NONE)
Secure Renegotiation IS NOT supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
Early data was not sent
Verify return code: 0 (ok)
trying to access the sites via Firefox results in:

SSL_ERROR_RX_RECORD_TOO_LONG

I wonder whether it would be wise to uninstall Apache, delete all in /etc/apache2 and /etc/letsencrypt and start all over again, installing apache, the virtual hosts and then registering the certificates again? Would that be possible at all?
And how could I avoid this trap:
"...Certbot's default method of adding a domain name to a previously configured cert."?

[MikeMcQ]

@Gantenbein

ssl3_get_record:wrong version

Looks like your Apache or comms setup has gone wrong. That error usually means an HTTPS request (port 443) to your system has gotten a reply from something that is "talking" HTTP (not HTTPS).

More generally, I have said about as much as I can given the lack of details you have provided. The output of the commands below are a good place to start for further assistance

sudo certbot certificates
sudo apache2ctl -t -D DUMP_VHOSTS

[Gantenbein]

Mike, you are my hero!

apache2ctl -t -D DUMP_VHOSTS
showed me that I accidentally disabled the two ssl-vhosts.
According to your hint I had already installed a dummy vhost. I then installed a self assigned ssl-vhost for that.
As it is now, someone who knows a.x doesn't get information about b.x and vice versa. Someone who enters the ip address just gets a warning:

Warning: Potential Security Risk Ahead
w.x.y.z uses an invalid security certificate.

The certificate is not trusted because it is self-signed

Thank you very much!