I think I have found a better solution. With a simple
.dokuwiki .wrap_noline {
column-rule: none;
}
I can write something like
<WRAP col2 noline>
...sections...
</WRAP>
Edit: This worked for a while. I successfully tested
<WRAP col3 noline> to rule out caching effects. At least I think I did. Because now it does not work anymore.
I'm confused :-)
This is what now seems to work:
.dokuwiki .wrap_noline {
column-rule: none !important;
}