I received an answer:
We are aware of the security warning displayed by the browser. This warning was triggered because Google Safe Browsing temporarily flagged our domain as a precaution, following the misuse of a publicly accessible resource within a Nextcloud instance by a single third-party user.
Please note:
* The affected content was removed immediately
* The responsible user account was blocked
* Our systems were thoroughly checked and no malware is present
* No customer data was accessed or compromised
*Our infrastructure remains fully secure and operational
TAB.DIGITAL is an official Nextcloud Gold Partner, operating according to strict security and compliance standards.
We have already submitted a review request to Google. Based on experience, the warning is usually lifted within 24–72 hours after the review is completed.