Hi. When I install the plugin WP Events Calendar (Version 6.6-beta) it works fine except for the widget. It seems that this plugin breaks the ability to drag widgets to a sidebar. When you deactivate the plugin everything works fine. Has any testing been done with this plugin? Any known bugs?

There is very little testing of plugins with Atahualpa because there are just so many it would be impossible to test each one and then every combination.

two things you can do,
1) update to the latest Atahualpa release (3.4.2 as of this writing...but be on the look out for 3.4.3) and test it there

2) try it with the default theme, does everyting work then

3) REPORT the problem to the plugin developer

As you point out, WP Events Calendar is beta