Hi Ricardo,
I’ve made a screen cast of your animation being played in my pc. Check it out here:
mediafire.com/?tdm025dyxvs (1 MB MP4 video format)
I have some comments on your file and the solution for reverse an animation:
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First try to use a lower size of the canvas for edit the animation in synfigstudio. It doesn’t make sense to have a 1920x1080 canvas in edition. You can change the output render format in any time at the render moment. So if you keep the aspect ratio you won’t have any problem to export it to any size later. I’ve modified the file to be 480x270 which has the same aspect ratio than the 1920x1080.
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When you share complex animations that include external files you have to set the external files paths to be relative to the current path of the sif(z) file that it is embedded· If not when placed in another computer you will have broken paths like it is the case. I’ve managed the two background files to be relative to the folder of the animation sifz file. I hope it works for you.
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I prefer to use the Offset Time parameter instead of a Time Loop Layer. You can do all the same things with the offset parameter and IMHO it is more intuitive. Unless you explicitly need a Time Loop layer for some reason, just encapsulate the looping stuff in a paste canvas and play with its Offset Time parameter.
As you can see in the attached file one of the window’s doors is playing properly. Notice how I converted the Offset Time parameter to linear. To play backwards at same speed set the ratio parameter to be -2s. It will delay two seconds of time per second of real time passed so 1s - 2s = - 1s what is the desired action. To let it play again normally just set the ratio to be 0s. Of course you have to tweak the displacement parameter to match the proper start and end loop. Look at the numbers and you’ll understand.
Do you dare to repeat the same with the other door? 
Here are the files corrected:
cen+objetos.zip
-G