I have a very similar issue. I love 1.7 (what I don't like in 2.0 is the narrow page look) - but seeing that it's end of life I decided to try and upgrade.
As I'm afraid of this change I'm doing it all locally on my PC before attempting this on the server.
1. Backed up my server DB and /data folder.
2. Installed 1.7.16 (the version I have on the server) locally and restored my DB backup and data folder -> 1.7.16 works OK locally.
3. Upgraded to 1.7.18 locally -> 1.7.18 works OK locally.
4. Upgraded to 2.0.18 locally.
At this point, going to the app index page gives me this error: