Hi @sbernhard
find the drawer.vm in the /skins/flamingo folder an edit it. search the line with <a href="$xwiki.getURL('XWiki.XWikiLogout', at the beginning. Edit the following and change the whole line:
<a href="$xwiki.getURL('XWiki.XWikiLogout', 'logout', "xredirect=https://KEYCLOAK-SERVER/auth/realms/master/protocol/openid-connect/logout?redirect_uri=ADDRESS_TO_BE_SHOWN_AFTER_LOGOUT"")" id="tmLogout" rel="nofollow">$services.icon.renderHTML('log-out') $escapetool.xml($services.localization.render('logout'))</a>
Afterwards no reboot is needed. Just reload a page. 
Logout will work without problems. I use this for some month now witout problems.
Remember to edit it again, after an update!