diff --git a/app/app.pro b/app/app.pro
index 8fe505c2..89ff5c0b 100644
--- a/app/app.pro
+++ b/app/app.pro
@@ -22,7 +22,10 @@ message("= MindForger QMake configuration ==========================")
 message("Qt version: $$QT_VERSION")
 
 QT += widgets
-QT += network
+
+win32 {
+  QT += network
+}
 
 mfdebug|mfunits {
   DEFINES += DO_MF_DEBUG
@@ -189,9 +192,11 @@ INCLUDEPATH += ./src/qt/spelling
 #
 win32{
     QMAKE_CXXFLAGS += /MP
-    !mfnoccache {
-        QMAKE_CXX = ccache $$QMAKE_CXX
-    }
+    
+    # DISABLED ccache as it causes compilation error:
+    #   "C1090: PDB API call failed, error code '23'" when used 
+    # when used w/ MS VS compiler:
+    # !mfnoccache { QMAKE_CXX = ccache $$QMAKE_CXX }
 } else {
     # linux and macos
     mfnoccache {
diff --git a/appveyor.yml b/appveyor.yml
index a4867e56..dcfe64e4 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -42,7 +42,10 @@ cache:
 
 # check https://www.appveyor.com/docs/windows-images-software for updates
 install:
+# QT used by: MF 1.55.*- (VS 2017 - change ABOVE if switching Qt)
  - set QT_DIR=C:\Qt\5.9.9\msvc2017_64
+# - set C:\Qt\5.12.6\msvc2017_64
+# - set QT_DIR=C:\Qt\5.15.2\msvc2019_64
 # - set QT_DIR=C:\Qt\5.9.7\msvc2017_64
  - set PATH=%PATH%;%QT_DIR%\bin
  - set M8R_HOME=%APPVEYOR_BUILD_FOLDER%
diff --git a/build/windows/build-win-installer.bat b/build/windows/build-win-installer.bat
index 5cde16f0..cdfeb514 100644
--- a/build/windows/build-win-installer.bat
+++ b/build/windows/build-win-installer.bat
@@ -28,3 +28,5 @@ C:\Qt\5.12.1\msvc2017_64\bin\windeployqt app\release\mindforger.exe  --dir app\r
 
 rem Build installer
 "C:\Program Files (x86)\Inno Setup 5\ISCC.exe" /Qp /DVcRedistPath="c:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Redist\MSVC\14.16.27012\vcredist_x64.exe" build\windows\installer\mindforger-setup.iss
+
+echo "Find installer in mindforger/app/release/installer/*.exe"
diff --git a/lib/lib.pro b/lib/lib.pro
index 035291d0..4f03d153 100644
--- a/lib/lib.pro
+++ b/lib/lib.pro
@@ -19,13 +19,13 @@ TARGET = mindforger
 TEMPLATE = lib
 CONFIG += staticlib
 
-#win32|macx {
-    # Qt Network as CURL replacement on Win - add Qt to libmindforger!
+win32|macx {
+    # Qt Network as CURL replacement on Win - add Qt to libmindforger
     CONFIG += qt
     QT += network
-#} else {
-#    CONFIG -= qt
-#}
+} else {
+    CONFIG -= qt
+}
 
 # Dependencies:
 # - INCLUDEPATH is used during compilation to find included header files.
@@ -56,9 +56,11 @@ mfdebug|mfunits {
 # compiler options (qmake CONFIG+=mfnoccache ...)
 win32{
     QMAKE_CXXFLAGS += /MP
-    !mfnoccache {
-        QMAKE_CXX = ccache $$QMAKE_CXX
-    }
+
+    # DISABLED ccache as it causes compilation error:
+    #   "C1090: PDB API call failed, error code '23'" when used 
+    # when used w/ MS VS compiler:
+    # !mfnoccache { QMAKE_CXX = ccache $$QMAKE_CXX }
 } else {
     # linux and macos
     mfnoccache {
diff --git a/lib/test/mindforger-lib-unit-tests.pro b/lib/test/mindforger-lib-unit-tests.pro
index b70ce89f..6fbdec67 100644
--- a/lib/test/mindforger-lib-unit-tests.pro
+++ b/lib/test/mindforger-lib-unit-tests.pro
@@ -17,6 +17,17 @@
 
 TEMPLATE = subdirs
 
+win32|macx {
+    # Qt Network as CURL replacement on Win - add Qt to libmindforger
+    CONFIG += qt
+    QT += network
+} else {
+    CONFIG -= qt
+}
+
+message("= MindForger library test QMake configuration ==========================")
+message("Qt version: $$QT_VERSION")
+
 SUBDIRS = lib src
 
 # where to find the sub projects - give the folders
@@ -26,4 +37,13 @@ src.subdir  = ./src
 # build dependencies
 src.depends = lib
 
+# ########################################
+# Diagnostics
+# ########################################
+
+message(DEFINES of app.pro build: $$DEFINES)
+message(QMAKE_EXTRA_TARGETS of app.pro build: $$QMAKE_EXTRA_TARGETS)
+message(QT of app.pro build: $$QT)
+message(PATH is: $$(PATH))
+
 # eof
diff --git a/lib/test/src/src.pro b/lib/test/src/src.pro
index b46c3564..e9137a1b 100644
--- a/lib/test/src/src.pro
+++ b/lib/test/src/src.pro
@@ -90,6 +90,11 @@ win32 {
 # compiler options
 win32{
     QMAKE_CXXFLAGS += /MP
+
+    # DISABLED ccache as it causes compilation error:
+    #   "C1090: PDB API call failed, error code '23'" when used 
+    # when used w/ MS VS compiler:
+    # !mfnoccache { QMAKE_CXX = ccache $$QMAKE_CXX }
 } else {
     # linux and macos
     mfnoccache {