I am having a similar problem...same error... "not able to write disc-at-once on an open disc". This occurs after i try burning to a disc in which i choise to continue a multisession disc. The session was imported, i added some files and selected Burn.
I have no problems other wise with my burner and this is the first time i have used the multisession feature with Nero. I have version 126.96.36.199 - the latest version.
I would love for someone to assist with this error,