Themes list failed to load

Hi

I’d like to change the appearance of your great plugin so it looks more flat and fits more into my website theme. So I clicked on “Themes” in the plugin navigation but than always an error message shows up:

Themes list failed to load. Please refresh the page to try again.
This also appears when I click on Addons.

Is there a possibility to get themes or check them out to see if there is a theme which fits into my site?

https://www.ads-software.com/plugins/brs-booking-reservation-system-woocommerce/