I have updated to /e/OS 0.18 a few days ago. Since then the microG Exposure API from thr Apps store is not installable on my FP3. Consequentially, the German Corona-Warn-App does not work either on my device, at least the risk assessment feature.
Any help appreciated,
Andreas
Regain your privacy! Adopt /e/ the unGoogled mobile OS and online services
Perhaps this here still helps? …
There is still an active bug that means Apps fails to install the Exposure Notification Framework. That defect specifes a workaround for that is
Workaround
If you are on /dev, download and install microG EN for dev
If you are on /stable, download an install microG EN for stable
All APKs are available at Releases · e / apps / GmsCore · GitLab
According to this response to the defect
Finally, I got it! A patch should be deployed early next week.
The root cause it microG is declared as a persistent app, preventing it to be updated.
I wonder whether that is the same ‘next week’ when we will hear about /e/'s exciting new branding 
Marvin, the developer of microg, has posted a workaround using adb or using a terminal app.
I have tried the adb solution, and it worked (at least on my phone 
)
According to Marvin, you seem to need root if you use the terminal app solution.
core-nightly.apk by the name you have given to the apk.adb reboot in the adb instructions does this).