Α Hoster said, that from the main domain (infopili.gr) it's not possible to open files that are found in subdomain (forum.infopili.gr). The only way is to define in the Appache settings an open_directory, in which the access will be allowed. For this I must tell them, in which directory of a forum your script need to have access. Can you tell me which directory of phpbb3 forum is this?

It will need access to the main forum directory... not sure how the feature he is talking about works... the main core phpbb files also load files in the includes directory and others... Do those need to be 'open' as well...

Not sure how your hoster is... but can you define the directory where a subdomain resides? For example on 1and1.com I can have a subdomain reside in a directory inside another domain and with that all the files are accessible.

In the end it might be easier to move the forum than jump thru a bunch of hoops

Finally I decided to move the forum in the main domain, because differently I believe that I will not achieve nothing. Ιn any case, thank you for your help.