Upload doesn't start

Before I update to 2.7 there were no problems. I uploaded all sizes of pdfs, but after the Update and the migration there should be a limit!?
If I want to upload a pdf, about 2 MB (the both php-variables post_max_file and upload_file_size are still big enough) the Upload doesn’t start. I cannot see anything, no error, no progressbar, no reaction. If I choose a smaller file (~1,3 MB) the process start immediately. Is there a limit? What can I do to analyse the problem???

thx in advanced