When running the new incremental backup feature I get the error
Warning: mkdir() [function.mkdir]: Invalid argument in C:\projects\VLE\Mobile Moodle\environments\xampplite-win32-1.6.6.exe\development\offlinemoodle-0.3\htdocs\moodle\lib\moodlelib.php on line 7756

At first glance it looks as thought mkdir() is passed an empty string or null value.