I am getting this issue this year not sure what update causes this. In the debug mode, I cannot check the values of variables in the watch window. They all show: identifier is undefined. I am currently using visual studio enterprise 2017 + Intel® Parallel Studio XE 2018 Update 2 Composer Edition for Fortran Windows*.
Someone told me to install the latest compiler? Does that help?
You are using compiler and VS versions that are old. Without a way to reproduce your issue, I don' t know if the newer compiler and newer VS solve it. The issue could also be caused by a change in your environment.
Updating your compiler and VS is certainly one option.