I've switched from Cyanogenmod to LineageOS using the experimental build and then flashing a nightly build afterwards. This worked great except for the fact that the phone and contacts app kept on crashing. I reflashed the nightly and -- to be sure -- this time I flashed the GApps immediately after as well.
I've tried both the Gapps "stock" package and the "micro", because I thought the problem could lie within Hangouts. It didn't.
When I boot my phone up, I get
android.process.acore has stopped working (or something along the lines of that; I'm using the German translation) and this message get's repeated everytime I try to open the phone, messages or contacts. All other apps seem to work fine.
Interestingly enough, I can get called. The aforementioned error message is displayed, however, I can tap on
Kill app without killing the conversation.