Problem with localized button images after uprading from 3.0.7-PL1 to 3.0.8

After upgrading from 3.0.7-PL1 to 3.0.8 in the following phpBB setup:

-Default Forum Language: Greek
-User Default Language: English
-Template is Prosilver and Prosilver-based Odyssey

I noticed that the localized button images do not display well. Specifically:

-If I do not log in and go to forum, I will see the button images in Greek (which is correct)
-If I log in with a user whose language is English, I will continue to see the image in Greek. The only way to get the proper image (ie English) is to hit refresh in the browser.
-If I log out, the images are not returning to Greek, but they remain in English from the last user session. Once again, I have to hit refresh button to get them back to Greek.

The issue is described in full here:

http://www.phpbb.com/community/viewtopic.php?f=46&t=2112518

I also recorded a video that I used a previous backup of my database (3.0.7-PL1 backup) where everything works fine and I upgrade this perfectly working installation to 3.0.8. After the upgrade the installation displays the problem described above. The links for the videos are:

Part 1/2:
http://www.youtube.com/watch?v=shbac19cnek

Part 2/2:
http://www.youtube.com/watch?v=sVhG2s4hpgk

The videos should be viewed in 480p since the original resolution is 640*480.

Full length video (WMV):
http://rapidshare.com/files/437881760/phpBB3.wmv

PS. In 3.0.6, there was a problem with button images. It was fixed in 3.0.7-PL1 but a new problem exists in 3.0.8).

PS2. I verified the problem with a clean installation of 3.0.8 (ie with a blank.default DB)