Nextcloud 22 - appstore is empty, no apps shown, cannot install further apps, cURL error 60

Hi everybody

May I ask once again for your help as I am still struggling with the appstore at my NC installation. It still shows no apps, I cannot install any app, there are just shown some standards (but not calendar etc. that I chose to install at the beginning.
Logs are printing

Warning appstoreFetcher GuzzleHttp\Exception\RequestException: cURL error 60: SSL certificate problem: certificate has expired (see https://curl.haxx.se/libcurl/c/libcurl-errors.html) for https://apps.nextcloud.com/api/v1/apps.json
Warning appstoreFetcher GuzzleHttp\Exception\RequestException: cURL error 60: SSL certificate problem: certificate has expired (see https://curl.haxx.se/libcurl/c/libcurl-errors.html) for https://apps.nextcloud.com/api/v1/apps.json
Warning appstoreFetcher GuzzleHttp\Exception\RequestException: cURL error 60: SSL certificate problem: certificate has expired (see https://curl.haxx.se/libcurl/c/libcurl-errors.html) for https://apps.nextcloud.com/api/v1/categories.json
Warning appstoreFetcher GuzzleHttp\Exception\RequestException: cURL error 60: SSL certificate problem: certificate has expired (see https://curl.haxx.se/libcurl/c/libcurl-errors.html) for https://apps.nextcloud.com/api/v1/apps.json

My ssl-certificate expires in October 2022.

I am running NC 22 on a hosted server at domainfactory in Germany.

I appreciate any hints for a solution.

Nextcloud version (eg, 20.0.5) : 22
Operating system and version (eg, Ubuntu 20.04) : hosted server at domainfactory
Apache or nginx version (eg, Apache 2.4.25) : I do not know
PHP version (eg, 7.4) : 7.73