Yes, Vivaldi generally works well with Let’s Encrypt certificates—it's built on Chromium, just like Chrome, so its behavior with SSL/TLS should be nearly identical.

However, the error you're seeing (“connection to the site is not secure”) usually points to one of a few common causes, even if your certificate seems valid. Here are some things you should check:

Check the full certificate chain
Let’s Encrypt certificates rely on intermediate certificates, and not serving the full chain is a common issue. Some browsers (like Firefox or Chrome) can fetch missing intermediate certs, but Vivaldi sometimes won't.

Make sure your web server is configured to serve both your cert and the intermediate (e.g., fullchain.pem, not just cert.pem).

Verify system trust stores
Since you're seeing the issue on both macOS and Linux, it might be that the version of ISRG Root X1 (Let’s Encrypt’s root certificate) on those systems is out of date or mismatched.

Update your system's CA certificates:

On macOS: make sure the system is updated and Keychain Access trusts the ISRG Root X1.

On Linux: run sudo update-ca-certificates (Debian/Ubuntu) or the equivalent on your distro.

Check for hostname mismatches or mixed content
Vivaldi may also throw security errors if:

Your SSL cert is for www.domain.com, but you're accessing domain.com without a redirect in place.

You're serving HTTP (non-HTTPS) assets on an HTTPS page (mixed content).

Check for HSTS preload or old STS headers
If your domain was previously submitted to the HSTS preload list or sent invalid HSTS headers, Vivaldi may enforce them differently.

The website works fine for me, please try with extensions disabled.
Also if you're trying to reach IP address please make sure it's reachable from your network first.

Thanks for the replies and for the recommendations about other password managers - I'm pretty established with both Vivaldi and with LastPass, so I would like to investigate solving this issue further before making big workflow changes / transferring to new services. I haven't had other significant issues in the last few years of use.

LastPass extension is up to date (tried removing and reinstalling too)

Vivaldi version is now 7.3.3635.12 (Stable channel) (arm64)

Thanks for everyone's advice, much appreciated!

Thanks for your answer.

@bsql61 When was the last time you restarted the computer?

Try temporarily removing it (what site is it?)

I found a solution. In the settings of vivaldi ctrl tab and ctrl shift tab are set to tab cycler forward and tab cycler backward. And in the next tab (by order) is ctrl pg down also in the "previous tab(by order)" it is set to pg up. So I changed them and it works flawlessly

I think this has gone away; I haven't seen that in the last month

@animdan Ok so I'm replying to myself since I just dabbled (like a too-eager kid) with custom theme creation and have already two minimalist versions (ProThemes): A dark version and a lite one. The dark version is awaiting approval as of this typing, while the lighter version is ready to be uploaded once the dark version is approved.

Gotta admit it cannot be easier than that, so thanks Vivaldi Team. Since I am GUI dev, I opt for minimalistic logical colour/shade arrangements, no distractions allowed! Correct me if I'm wrong, but I haven't yet found customization options at the Bookmarks panel level. Next time I need to post something, I'll make sure to do my home work so as not to bother you lot LOL 😉

Keep Up The Good Work!

@joeker Hmmm. I just now switched to Vivaldi after trashing Brave because that browser resets all settings to defaults after a few hours of usage (latest current version) - an enormous pain in the behind I tell ya.

I use iMac Intel 2020 on Sonoma and don't want to update to Sequoia because that system version is more adapted for Apple "silly-cone" architecture, thus adding useless resources on my Mac if I installed it.

So I guess Vivaldi for Sonoma is what will work for me, at least for now, based on your observations.

The finder works for me without crashing.

@snowcreative Thank you!!!

@sgunhouse Thank you for the response.
After reinstalling Vivaldi, I logged into my same account using the same username and password and sync the lists, nothing appeared and now the message I am getting is "Sync completed" and both the reading list and bookmarks are blank.
I don't know where else I can retrieve those information. I have iCloud account and one drive but they don't back up Vivaldi lists.

Please let me know, if you know where I can access those lists in my MacBook Air- if possible.

Yes, that is what I meant. That's a bummer. I hate they got rid of it. Maybe it was inadvertent and they'll revert it. I'll add it to the feature requests section. Thanks for the reply.

@StrobeT4 I tried to use brave again for the last two days, and I quickly came back to vivaldi 😁

@ThePfromtheO 😆

Still looking for help on this.