[Dec 02 11:03:07 UTC] Open failed for 'D:\Program Files\distributed.net\buf ...
                      File is not in distributed.net client format.

then you need to delete one or more of your buffer files. Version 2.9 of the client mandates a new file format and will refuse to read buffers that were created by an old version of the client.

If you have completed blocks in your old buffers, you should consider flushing them with an old client before deleting them (and before upgrading to the new client).