First of all thanks in advance for your help.
Now about my problem: I installed through "Plesk Parallels" WordPress on a subdirectory (/wordpress). Then i followed the official instructions from WordPress in order to make my site appear when someone goes to example.com and not to example.com/wordpress.
All working till I tried to change/activate another theme.
Whenever I activate a theme I get an 404 error page.
I would appreciate your help on this cause im sure this error will occur when I'll try and change stuff like plugins etc.
Thanks in advance for your time.