due to an update of the dom structure of displayed events, the way how a category-styling can be achieved has changed slightly.
instead of using
div .fc-event.category-3,
div .fc-event.category-3 a {
background-color: red;
border-color: red;
}
you should now apply the style for any child elements. just replace the a by an asterisks:
.fc-event.category-3,
.fc-event.category-3 * {
background-color: red;
border-color: red;
}
BUT, you cannot use your old (theme-specific) fullcalendar.css anymore. You have to upgrade the fullcalendar.css, copy the plug-ins fullcalendar.css again to you theme root folder and then apply your styles again. I think this should fix your header problem.
One Note about your lost events. The update does not delete any events. But the database fields from and to are deleted and added again with different names, because of keyword-conflicts. In your case, deactivating and activating the plug-in again should have done it, because a migration is triggered when activating the plugin.
The 'final' V1.5.1 will just rename this 2 colums, which should not cause any errors.
Good night