Subdomain problem

[ferges]

I installed Prestashop 1.2.2 in a folder on my server. The path to the shop is www.mydomain.com/online. I put this "/online/" into 'PS directory'. Next I linked this folder to virtual subdomain to access my shop directly from address www.online.mydomain.com. If I leave folder "/online/" in the preferences tab, everything work smoothly except the links are www.online.mydomain.com/online/. If I change PS directory from "/online/" to "/", the problem with double "online" in links is gone and the front office works correctly, but there are some problem in the back office. Firstly, I can edit my products, but I am not able to access any tab after images (actually tabs are empty after clicking them). Secondly, the images to change back office language are gone. Any help? I don't know, if this is a Prestashop problem or the redirection on my server is faulty. I'm not sure if this has any importance, but when I access back office through www.online.mydomain.com/my-back-office-folder/, I'm redirected to www.mydomain.com/online/my-back-office-folder/

I will appreciate any help.