Its tough to suggest exact solution as I cant see the actual problem and also cant reproduce the problem in my environment.

First check you don’t have any javascript conflict issue. If you able to access database check that Advanced post slider database table ‘your_database_prefix_advps_optionset’ and ‘your_database_prefix_advps_thumbnail’ created successfully with default data.

Do you have a default slideshow under each template and default thumbnail size under thumbnail tab?