There are unresolved project references.

Problem:
When I open my old projects in Visual Studio Enterprise 2017 with Resharper plugin, I get the following warnings:



Please note that the problem does not take place for new projects. But somehow it still occurs when I remove and recreate my old projects.

The problem is inconsistent when I disable Resharper. Warnings sometimes show up, sometimes don't.

What I've already tried:
- Reinstalling everything (Removing Unity, VS, R# and installing them all over).
- Removing everything except Assets folder in the project folder.
- Cleaning Resharper cache.
- Re-creating the whole Unity project.
- Clicking the middle option on the prompt (Let Resharper call MSBuild... option). It does not help.

The problem persists. Any help is appreciated.

 

This message in another thread solved my problem. You might want to read the thread from the start:
https://forum.unity.com/threads/visual-studio-problem.507850/#post-3317346