go to the address:
http://yourmoodleroot/head/admin/tool/filetypes/delete.php?extension=aa&theme=clean

Switch to clean if you are still using a different theme.
Look at the alignment of the 'Yes' and 'No' buttons.

This is a general problem related to $OUTPUT->confirm.
The issue was recently introduced modifying the file:
head/theme/bootstrapbase/style/moodle.css
The wrong code is: margin:0 0 10px 5px; about column 152107