User clicks on the Modal launch button, the modal opens and then closes leaving the gray layer blocking the screen. After that, the gray overlay will block any further interactions with the coach.
Usually, this behavior doesn't present any client-side or server-side errors.
Several things can cause this behavior, but usually it is related to Modal or other Coach View boundary events. During the boundary event action, some variables is either changed or removed which causes an update to the coach which in turn breaks the modal.
A sample case stems from using the same variable by multiple Coach Views. For example, it is possible to use the same String variable as the 'Data Retrieval Service Input' option of an AJAX-populated Table and the 'Boundary Events > Action Executed' of a Modal. In this case, when the Modal opens the variable gets updated with the boundary event action but the change to this shared variable will also trigger the table control's AJAX service and its change event. This content update ends up breaking the Modal references.
Resolving the problem
Make sure there are no boundary events or other actions changing the Coach View that holds the Modal. Use isolated variables for each Coach View unless the cascade of actions is really what is intended. Avoid changing the table binding while the Modal is open.