Uhhh .... not good ... at all. One of the first things that take place with anyone trying to login is a sessions file in /moodledata/sessions/. IF, hopefully, this is not the case, but could be the disk/partition where /moodledata/ is located is full, out of space - cannot write a small sessions file.
Since you've been having issues with restoring a 6Gig backup, could be that there have been sooooo many failed attempts that /moodledata/temp/backup/ has filled up disk space. So if you have anything in /moodledata/temp/backup/ manually erase any/all in there. Manual removal will not hurt anything.
If your server is an all-in-one and db server is on same server, your databases are in danger ... DB servers don't like running out of space. DB's can get corrupted.
'SoS', Ken