I created a simple Visual Fortran 2018 console application from the VS2017 wizard. After upgrading Visual Studio from 15.8.4 to 15.9.4, the Intel tools were inaccessible and I could not reopen the solution.
I deleted the VFPackage folder in VS install directory and reinstalled Intel tools, as documented here and it fixed the integration issue:
However, opening the solution still shows "incompatible". I create a new solution with the same wizard and it works. I compared the .sln and .vfproj of the two directories and I cannot see any significant difference.
The solution is to remove the directory .vs in the source directory. There was some stalled value that prevented the solution to load correctly.