1. Update Office to the latest version. With Office updated, some issues would get resolved. So it’s a good idea to keep Office up to date.
2. Perform Office Online Repair to fix the problem.
3. Turn off the VPN and proxies if there’s any enabled.
4. Scan your computer with Microsoft Safety Scanner, which would help us to get rid of viruses, spyware, and other malicious software. The Microsoft Safety Scanner is a free security tool that provides on-demand scanning and helps remove viruses, spyware, and other malicious software. It works with your existing Antivirus software. Refer to the link to download and run the scanner.
5. There’s a knowledge-based article from Kaspersky that indicates some possible causes, you can refer to steps listed there to remove the certificate warning messages.
Regards,
James
