[Plugin: Booking Calendar] The widget doesn't show up right in the sidebar table

Hi all,

I installed this plugin, and it works very nice.
That’s why I’m trying so hard to solve my next problem:

When I enable the calendar widget (or the form) in the sidebar, there goes something wrong. You can see it here: Click

The widget doesn’t show up right in the sidebar table. It has to show up between the borders of the table. But it doesn’t.
All the other widget which I tried are doing this well.

I know there will be a simple solution, so I hope someone can help me out.

Thanks in advance.
Greets, Mark

https://www.ads-software.com/extend/plugins/booking/