If you enter some "quoted words" in the description filed for the subscribe widget, then this description will not be displayed.
The problem lies inside
class-sendpress-widget-signup.php at line 62. The functions tries to build attributes for the shortcode, but forgets to escape possible quotes and other delimiters in the arguments from $instances.
I also wonder why
do_shortode() is used for own shortodes. Why not just call the shortcode function
load_form() directly? No need to build attributes string, just to parse the same string in the next step, i would think.