Nothing yet with this one @Ermin Trevisan??. I will look into this one when I have time if there is no solution yet

Unfortunately not yet. I believe in more complex situations with different databases, domains and subdomains as we have, it would be really helpful to know what headers Odoo receives from the proxy server in order to sort out any problems.

By the way, I'm trying to use caddyserver as my proxyserver.

I'm about to finish my integration with caddyserver in our Odoo deploy manager. Looks pretty good

My questions for example are: do I need the dbfilter option, what happens when I'm using the dbfilter_from_header module, what happens if I have configured both etc.

both?

The dbfilter option for handling subdomains (database_x.twanda.ch) and the dbfilter_from_header module for customers with their own domain (www.customer_x.ch) was an idea that came to my mind...

It should be www.database_x.ch (not www.customer_x.ch)

Not need for that, your dbfilter actually it's a regular expression that could be used to match several dbs, it will have replaced %d and %h with subdomain and host values, for example
^%d$
but also could be:
^%d|%h|database_x$

I did not think that way, interesting. But to make sure I understand it properly. What I would like to achieve is the following (examples): http-request: www.domain1.ch or domain1.ch should go to database domain1, request www.domain2.ch or domain2.ch should go to database domain2, request subdomain1.twanda.ch should go to database subdomain1 and so on. If a request is made to a database that does not exist, it should go to www.twanda.ch.

You could setup a redirect for your www.twanda.ch virtualhost that have the correct config for the dbfilter in the specific instance