Ok, I’ve been using Davdroid with a self signed certificate that was imported by CADroid. Everything worked well up to the upgrade to Nougat.
Davdroid shows a screen that the certificate was issued by an unknow CA if it starts to sync. In the windows (that covers ca 3/4 of the screen) it shows the certificate details. Davdroid asks if it should trust the certificate. It gives the options “cancel”, “once” or “always”. Answering “always” makes it vanish.
I don’t really know if it is an app window of Davdroid or a system message. In the task manager there’s Davdroid in the window title.
It always shows up twice, I suppose for calendar and contacts. I have one calendar and one addressbook to sync with a Baikal server.
Androids account settings shows that there are synchronisation problems in the davdroid account. Last sync is from the day when I upgraded.
I don’t have the problem with my phone that runs Marshmallow. So I also think it comes from Androids new policy of handling certificates.
I am not a developper but I think there are some changes that have to be made in Davdroid.