Ok, I found the problem.
There is a bug (at least it's a bug for me) in the system, obviously (that is the only plausible explanation) the CSS definitions of the WRAP plugin are integrated into the global CSS file (the entire CSS cascade)
after the content of the USERSTYLE.CSS. Therefore, it is not enough (as usual) to create a selector with the same specificity. You can cover up the problem with an
!important (not a nice method but it works):
.dokuwiki .wrap_col2, .dokuwiki .wrap_col3, .dokuwiki .wrap_col4, .dokuwiki .wrap_col5, .dokuwiki .wrap_colsmall, .dokuwiki .wrap_colmedium, .dokuwiki .wrap_collarge {
column-rule: none !important;
}
Edit: Ok, it is not a bug. Rather, it has to do with the different blocks within CSS. See below.