ERROR: Unknown: errno:2048 Only variables should be passed by reference

Trying my first Dropbox backup. Everything looks okay however files never reach my Dropbox destination. I have the following error in the log:

2012-10-16 11:26:32: finish: snapshot-1350408366-121016-172631-5fd90702.zip

2012-10-16 11:26:32: Sending file: /home/myuser/public_html/wp-content/uploads/snapshots/snapshot-1350408366-121016-172631-5fd90702.zip

2012-10-16 11:26:32: Destination: dropbox: Dropbox

2012-10-16 11:26:33: Sending to Dropbox Directory:disappointed:Backups/Web Sites/

2012-10-16 11:26:33: putFile file success: snapshot-1350408366-121016-172631-5fd90702.zip

2012-10-16 11:26:33: ERROR: Unknown: errno:2048 Only variables should be passed by reference /home/myuser/public_html/wp-content/plugins/snapshot/lib/destinations/dropbox/includes/PEAR_Includes/HTTP/OAuth/Signature/Common.php on line 58

Please help.

THANKS!