After creating a project, I was able to Build it an create an EXE. Upon saving it, closing C::B and then coming back it later, I tried to Rebuild it. I immediately got an error. Upon searching the net, I found some people talked about making sure the Toolchain Executables was set up properly. I also saw someone mentioned they renamed a file then renamed it back and it worked OK. In my case, I renamed gfortran.exe to gfortranb.exe. Then I renamed it back and it Built just fine. The next time this error came up, I simply went into the Toolchain Executuables menu (did not change anything) and I was able to Rebuild.
So my guess is that somehow it "forgot" what was in Toolchain Executables by going into this menu it forces an update? I had subconsciously just used this workaround (it happened maybe 5 times - not sure if it was random). But BlueHazzard is helping me with something and seemed to run into an issue that I suspect is related this this behavior. I might be able to create a video of it or something...provided the behavior is consistent.
