Here is an other forum post that may be of some help.
And basically the exact same thing again
I just did a search on google for "site:phpbb.com Warning: Unknown(): open_basedir restriction in effect. " and that's where thoes links come from. It's about 15 hits that all say the same exact thing.
Drexion, a Support Team Member at the phpBB forum said this in one of the above links:
"The 'open_basedir restriction' is a hosting restriction. There may be another reason, but some hosts use it as a means of helping to prevent malicious scripts from being run. (especially in certain directories)
Basically the folder which you have uploaded the phpBB forum to has this restriction in effect - there is nothing we can do, you have to contact your host for support. (or possibly move the 'phpBB2' folder to a location not under this restriction?)"
I guess you should contact support again, showing them where the error is displaying (with a link) copy and past the entire error into your support ticket. and then provide them with the information here (in a link again) as well as putting the above quote as text with the ticket.
Hopefully they will be able to solve it this time for you. Perhaps, the sarge upgrade, for some reason, made this funny little change on your machine.