Prestashop internal server errors

[taha116]

So im having an apparently not so uncommon issues with prestashop.
I cant log in as a user more then once. After my initial log in if i sign out and try again it says this page is temporarily unavailable. I did make another account and test it and still had the same problem

Also, when I have prestashop install some of its modules it gives me an internal server error every time I try using them as a user. For example the refferal program module, each time i do make an account and sign in and click the link that is added to the top left I get the internal server error page.

While typing this I found this :
http://x10hosting.com/forums/free-hosting/131658-500-internal-server-error.html

To do what Anna did, I just set all folders to 755 and all other files to 644?
Would this really solve the login issues as well as the internal server errors?

thanks for your help