Can't say for sure, but I always recommend you upgrade in order ... not to jump three versions that is. There are some major changes between 18 and 19 right from the start if I remember correctly.
If you did jump directly from 17, I would download the very last versions of 18 and 19 from the top of their boards. Remove version 20, then install 18, then 19, then twenty again. I always make a final backup of the library before each version upgrade, but this might be overkill. You version 20 license allows you to install the older versions. Let the program do its dance right from 17 (still installed on your PC?)
Now that might not be it, but the options are no longer centralized (every audio zone has its own config starting with 19), audio analysis totally changed which adds new fields to the database etc. ... and it doesn't take long to do. Oh, btw I wouldn't remove the older versions, until you have 20 up and running and if they are all set to boot on startup (including the media server) turn that option off and make sure old shortcuts are not on you desktop so you are not loading one of the older versions by mistake.
After the new version runs ok, then you can get rid of the older versions if you want.
Hope this sorts you out