BuddyPress.org

You could override the template and insert your theme’s sidebar. Depending on which Buddypress template you’re using, you’ll find it in

plugins > buddypress > bp-templates > the theme you're using > buddypress > members > index.php

Once you’ve grabbed it and inserted the sidebar there should be plenty of tutorials on the net on how to upload it to your own server, e.g. https://wproot.dev/blog/override-buddypress-template-files/

This isn’t a Buddypress related problem but an issue with your current theme / server.

A quick look through your site shows you’re using a bloated page builder, use redundant styles and scripts not enqueued properly for specific pages, dont minify your js / styles and many other things.

You’d need to clean that up and take it from there.

https://starkehand.org/datenschutz/ never calls your theme’s sidebar. You need to put that in yourself or ask the developers of the Neve theme to do it for you.

@stellamariee Go to Pages > All Pages then find your datenschutz or members page and click on the Edit link under that. Click on Page Attributes under Documents in right hand sidebar. Under Template, see if you can change it to something like Page with sidebar or something similar. Sometimes, themes have default layouts of Page with no sidebars, so if you want sidebars, you’d need to set it to that page template. Good luck.

@rootcr8r it’s theme-specific, try to inspect it.
https://developers.google.com/web/tools/chrome-devtools
When you want to work with the DOM or CSS, right-click an element on the page and select Inspect to jump into the Elements panel. Or press Command+Option+C (Mac) or Control+Shift+C (Windows, Linux, Chrome OS).

@benhutchenspearson Template Hierarchy is an excellent point to start to get a better understanding of BuddyPress template files.

Template Hierarchy

@laschwi try with a default theme ( 2020 ) to make sure it’s not theme conflict

Did you try deactivating all your plugins other than Buddypress and using a standard theme?

If you’re using the same server where uploads were possible before its most likely a plugin / custom code bugging it.

Otherwise, sometimes GD-library can be installed by not activated on the server.

Use this hook which is available right after they join.
do_action( 'groups_join_group', $group_id, $user_id );

Try this in your theme/functions.php or in bp-custom.php:

function schene_promote_group_member( $group_id, $user_id ) {
    $group_member = new BP_Groups_Member( $user_id, $group_id );
    $group_member->promote( 'admin' );
}
add_action( 'groups_join_group', 'schene_promote_group_member', 10, 2 );