On my Nokia 5, I've been having issues that I just can't fix, specifically with Microsoft's Cortana app. The first time I installed the app it worked fine. Then I did a reset on my phone because I was having a problem with SMS messages and after the reset, Cortana didn't work anymore. It crashed instantly after asking me to log in, and it asked me to log in again every time.
A couple days ago, I got tired of it and I reset my phone again. I installed Cortana after it came back to life and it worked fine. For one day. Now it's back to the exact same problem. It doesn't matter what version of Cortana I install, it just doesn't work. This has to be a device issue.
But I have another problem that popped up after this last reset. Microsoft's Outlook app won't show my Outlook contacts. This could be a Microsoft problem and I sent feedback to them already, but it seems weird because this worked before.
You should really question Microsoft about this issue :D
I haven't tried cortana. I probably won't until the timeline feature comes on android and we're forced to use cortana. However, I suggest you clear the Cache and Application data and try once again.
You can also check Settings > Accounts > Office / One Drive / Microsoft and remove it before re-installing the application.
Outlook application has people tab to display contacts. Recheck it after installation.
If you have installed Google Assistant, it could be interfering with Cortana. As a temporary check , remove that application and try Cortana alone.