Are there any indications why the server ann.nl.tab.digital is not accessible?
Hey @modi welcome to the Nextcloud community! 
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Please refer to their support centre https://support.tab.digital/nextcloud or contact their support for issues with that service.
https://support.tab.digital/nextcloud
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This server does not exist (no DNS entry):
https://www.whatsmydns.net/#A/ann.nl.tab.digital
Perhaps some maintenance/updates on the infrastructure. Many providers inform you when they plan service interventions, or at least have a status page of their services. Or their twitter/mastodon whatever.