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Detailed description of the issue. Put as much information as you can, potentially with images showing the issue.

Steps to reproduce:

1. have calendar configured to poll nextcloud and google accounts for calendar details (I have both - but this error seems specific to the google connector). This would have actually been set up via gnome-online-accounts (I'm using Ubuntu-18.04)
2. connect to (say) a hotel network where you have to authenticate via a hotspot/portal before gaining Internet access
3. gnome-calendar appears to detect that it's attempts to connect to the backend calendaring services results in seeing a cert that isn't correct (eg its outbound connections to google.com port 443 were intercepted by the hotspot until I successfully authenticated/etc) - so it sees the hotspot cert instead of google.com
4. days later after leaving that particular hotspot network, I see random "cert error" popups telling me an invalid TLS server is involved. eg right now I'm at home on my WPA2 Wifi network (ie no portal/hotspot) and I'm seeing popups from the hotel hotspot I was last using days ago... I can't even recall seeing those errors when I was in the hotel - where they would have made much more sense. I'd give you a screen shot but I closed them - and they don't seem to recur on any consistent basis - but they will come back,,,

BTW it took me days to figure out that gnome-calendar was behind this. I also the same calendar with Chrome and Thunderbird/Lightning so was wondering what application was behind this as the error popup doesn't mention an application name. In the end I had to run "xwininfo -a" and click on the popup for it to tell me the PID - and that pointed to gnome-calendar. You really should put the app name in the popup details...