When using the http upload, if I upload too many files at once the upload fails and I get the error "You do not have proper rights to access /zp-core/admin-httpupload/uploader.php.".
Shouldn't the error be something like "your upload timed out, please try uploading less images at once"?
P.S. 100th post confetti
What browser are you using for the upload?
About the only thing I can think of is that the "post" size is limited and some data is not being passed during the upload.
There is a parameter passed with the upload indicationg what user did the upload. If it is lost, then you will not be "logged in" for the upload and will get that error.
We can probably add a test for this and give a different error. Probably best you add a ticket as I am not sure I will be able to reproduce and test this, so will need your help.