Hi Eran,
I've had this problem before, this issue should be fixed in the latest release of studio pro (9.9.1), so if you have the chance it would be ideal to upgrade your project.
If you do not have that possibility right now, you can re-publish your project, by hitting the publish button next to the play button and it should run again in a few minutes.
I have heard this before. I would file a support ticket for that ( https://support.mendix.com/ ).
Regards,
Ronald