Hey @Eifelkuno welcome to the Nextcloud community! 
You have a hosted Nextcloud instance at TAB.DIGITAL. The Nextcloud community is not affiliated with TAB.DIGITAL.
Please refer to their support centre https://support.tab.digital/nextcloud or contact their support for issues with their service.
if you have specific questions to the Nextcloud community pertaining to Nextcloud in general feel free to post using the provided Support template. Please use this when you request support or consult Nextcloud documentation.