Hi,
This error message should pop up if the upload-test file (chkXXXXX.tmp) could not be successfully created. In your case the .tmp file does get created, so it seems that the error message is false. Try to upload your files. It might work properly. It is possible that the test login gives you a false message, but the upload will be successful anyway.
HTH,
Greg
Gergely Kmethy
VP, Customer Success, Graphisoft