Adobe Media Encoder failed to encode with the following exception : TypeError: null is not an object

Hey,

I'm a new user to Adobe Animate, making really simple animation, no coding or scripting, just using the motion tween options and masks. I tried exporting the project with the "Export Video/Media" option using the format H.264, yet I keep getting this error:

"Adobe Media Encoder failed to encode with the following exception : TypeError: null is not an object"

I succeeded exporting that way other projects in the past. What could be the issue?

Please help.

Thank you!