Problem with interface in PM

There is a problem with interface of PM which is expressed only at one user on my forum.
That interface cannot fit him normaly on the whole screen, because suddenly (it was ok before) it has become too, too wide, and the whole interface is moved too right, so if he wants to write a message to someone he needs to move his scroll in right, but it's not normal how are some parts of that interface away from each other.
If I try to send that user a PM the problem on my screen, and with that interface becomes the same.

Every other user has normal interface!

How to solve this problem?

hello,

can i have a screen shot to show the problem to the technicians!!?

Yes of course!
Because the interface is too wide, I had to make several captures and they go by this order:
https://2img.net/h/i186.photobucket.com/albums/x94/etro_photos/PM1.jpg
https://2img.net/h/i186.photobucket.com/albums/x94/etro_photos/PM2.jpg
https://2img.net/h/i186.photobucket.com/albums/x94/etro_photos/PM3.jpg
https://2img.net/h/i186.photobucket.com/albums/x94/etro_photos/PM4.jpg
https://2img.net/h/i186.photobucket.com/albums/x94/etro_photos/PM5.jpg
(Take notice where is position of horizontal scroll)

And this is the interface of other users' PM (normal interface)
https://2img.net/h/i186.photobucket.com/albums/x94/etro_photos/NormallookofPMinterface.jpg
As you can see there is no horizontal scroll at all.