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.).