I am also having the same issue.  Receiving the following errors during file transfer:   
552 Disk full - please upload later 
Error:	Critical file transfer error   
According to the stats I only have used 0.36 / 250 MB space.  I'll be keeping my eye on the thread, just thought it was best to let you know.   
Cheers,   
John   
(P.S. - Just noticed it can transfer the folders fine, but when you try and transfer a file, this is where the critical errors occur.)