problem is fixed

The feed shows up fine viewing the file in Firefox.

Looking at your feed validation results, it appears that you have some “hish-ASCII” characters in the text of your first entry.  You can replace the high-ASCII character with the appropriate entity.

problem is fixed

But how do I replace this sign so that it does show?

When you create (or edit) your entry, instead of directly using the “é” character, instead use the numeric entity for it: “é”.

On top of that I really don’t know how to fix this warning: “Your feed appears to be encoded as “UTF-8”, but your server is reporting “US-ASCII” [help]”

If you’re on an Apache-based webserver, then you can create a blank text file in the main site directory on your server named .htaccess (the period at the beginning is important).  Inside, place this:

Is pMachine supposed to update the xml-file after each new message that is posted with it?

Yes, pM Pro will normally update the feed file whenever you post a new entry or edit an entry.  In order to be able to do that, the file ("index.xml", in this case) must be set to file permissions 666 or otherwise set as “writable”.

problem is fixed

The “.htaccess” file should be saved as a regular text file (i.e. ASCII text, not as BINARY), but it should not have a “.txt” extension.  It should be named [color=red].htaccess[//color] exactly.  Saving as UTF-8 should be fine.

When you upload the “index.xml” file it doesn’t really matter if it is blank or not.  So long as you set the file permissions correctly so that pM can write to it, the file contents will simply be deleted and recreated each time, which is the correct behavior.

in FireFox it now says that “the XML-document doesn’t seem to be associated with any style-information”.

That’s expected.  It’s not really an “error” since almost no RSS feeds will have style information associated with them.  Just ignore “info warnings” like that when viewing directly in a browser.

problem is fixed

The .htaccess file was to take care of the Your feed appears to be encoded as “UTF-8”, but your server is reporting “US-ASCII” error.

If adding the file caused problems then you should contact your Host/server admin and discuss that.

problem is fixed

Put the “.htaccess” file in the same directory as the “index.xml” file.