Plugins crashing with "No module named qgsprojectionselectionwidget" in Windows?

Probably your QGIS is trying to import your widget from .h file like in c++.

Please open your .ui file and find lines:

<customwidget>
   <class>QgsProjectionSelectionWidget</class>
   <extends>QWidget</extends>
   <header>qgsprojectionselectionwidget.h</header>
  </customwidget>

Then in your header tag change to:

<customwidget>
   <class>QgsProjectionSelectionWidget</class>
   <extends>QWidget</extends>
   <header>qgis.gui</header>
  </customwidget>

Change the same tag for all your widgets with error (qgsmaplayercombobox etc.).