Your browser on your PC not being able to reach your server won’t be reflected on your server’s logs.
If you have an issue with Linux Mint, you’ll have to fix it on Linux Mint.
Within our guides that show you how to trust the Root CA on Linux there are a couple of warnings related to using flatpak apps. You should probably check that you saw those first, if you’ve taken that into account, I can suggest a couple of other things.
Had a bit of delay on my end, but ready to tackle the issue.
When trying to open the nextcloud UI on Firefox i do get the notification: please use the command line updater because updating via browser is disabled in your config.php