just want to note here, I had the same issue on an installation reachable with 3 subdomains on the same webspace. Everything worked fine while the url was the same than on installation. Problems as described above came up after the first call with the new subdomain. no login possible (resolved by delete all session files), errors about the unwritable img path on saving entries. the solution for me was to chmod 777 /assets, /session, /cache, /files and all it`s subdirectories. safe_mode is off. maybe this has something to do with sessionFingerprint and sessionChallenge, but haven`t looked further in this because everything works fine now.