Hello Veronica and welcome aboard!

You made the right choice by choosing Elgg for your social app.

Is Elgg still updated and "taken care of"?

Elgg is constantly updated and supported:
https://learn.elgg.org/en/5.x/appendix/support.html

This is how it happens on GitHub https://github.com/Elgg/Elgg/commits/master

I see a lot of posts hundreds, if not thousands, of days old

In fact, Elgg as a framework is stable and reliable, perhaps it lacks a good user interface, but there're also developers who will help you improve your app for free or for a fee.

The decrease in activity occurred due to the fact that more and more Elgg developers “grew up” and Elgg as a free and open source product rests only on enthusiasts.

There's a Telegram channel that regularly published info about all Elgg plugins and releases, as well as other useful information. Subscribe here: https://t.me/elggnews

There's chat also where you can ask questions just like in this Community: https://t.me/elggchat

unfortunately I can't seem to attach a screenshot

This community aka forum is made on the basis of Elgg version 2 and the administrators simply don't have enough time to update this site: https://elgg.org/discussion/view/3290256/elgg-community-future

That's why many things here still look strange ;)
 

is there any way I can upload an Excel-file to a group

By default, activating the "Files" plugin allows users to upload any files, including Elgg.

In the future, they can only be downloaded.

There's a 3rd-party plugin lets users preview MS Office files (doc, docx, xls, xlsx, ppt, pptx), Apple iWork pages, Adobe eps, and zip files.

I've never used it but you can download the version for ElGG 5 here: https://github.com/rohit1290/gdocs_file_previewer

 and then let the group members edit it right there?

There are no plugins for collaborative document editing for ElGG 5.

For this you need to hire a developer ¯\_(ツ)_/¯

https://elgg.org/groups/profile/75603

I've never heard of such a problem in Elgg but there're a few Q&A on StackOverflow. 

Basically, this issue is related to PHP session or php.ini configuration: 1, 2.

Do you have any custom settings in php.ini? 
Are there any other errors in the server error logs? 

You can also look at the Elgg settings if you need something special. 

For example, I recommend using a custom session name but not 'Elgg':

$CONFIG->cookies['session']['name'] = "YourAppName";

Hello, 

thanks for yout reply. No, there Aren't any specials in php.ini. 

We just tried it with composer. But there, we reach our memore-limit:

First try with 512M instead initially 256M:

XXXXXX@ssh:/var/www/XXXXXX/html/ELGG$ php -d memory_limit=512M composer.phar install
No composer.lock file present. Updating dependencies to latest instead of installing from lock file. See https://getcomposer.org/install for more information.
Loading composer repositories with package information
Info from https://repo.packagist.org: #StandWithUkraine
Updating dependencies

Fatal error: Allowed memory size of 536870912 bytes exhausted (tried to allocate 8192 bytes) in phar:///var/www/XXXXXX/html/ELGG/composer.phar/src/Composer/DependencyResolver/RuleSetGenerator.php on line 68

Check https://getcomposer.org/doc/articles/troubleshooting.md#memory-limit-errors for more info on how to handle out of memory errors.

Second try with 768M

XXXXXX@ssh:/var/www/XXXXXX/html/ELGG$ php -d memory_limit=768M composer.phar install
No composer.lock file present. Updating dependencies to latest instead of installing from lock file. See https://getcomposer.org/install for more information.
Loading composer repositories with package information
Updating dependencies

Fatal error: Allowed memory size of 805306368 bytes exhausted (tried to allocate 41943040 bytes) in phar:///var/www/XXXXXX/html/ELGG/composer.phar/vendor/composer/semver/src/CompilingMatcher.php on line 92

Check https://getcomposer.org/doc/articles/troubleshooting.md#memory-limit-errors for more info on how to handle out of memory errors.

Elgg installation via composer:

composer self-update
composer create-project elgg/starter-project:dev-master /var/www/XXXXXX/html/ELGG
cd /var/www/XXXXXX/html/ELGG
composer install

What's OS on your server?

No edit option to edit the above.

All Ok.

Don't worry about the warnings, it's normal ;)

Thanks. Just wanted to make sure I wasn't seeing anything bad :).