The linked webpage opens for me although I do not understand Greek.

Try clearing the browser cache and the cookies for that site then try again.

• to clear the cache use Tools -> Clear recent History ->
  and select everything as the time period (but do not select other items initially only cache.)
• to clear cookies see deleting cookies you may want to clear cookies for that site alone
• for other ideas see Fix login issues on websites that require a username and password

Besides clearing the browser cache, have you any security or privacy software that may be blocking that site or its scripts consider temporarrilly disabling any add blockers and if you trust the site temporarilly disable any AV etc software .