Strange uploading process

Hello guys, I have strange uploading behavior. When I try to upload few files via FileReference#upload(), the following occurs:

  1. only 2 files can be uploaded at the same time
  2. if uploading of other files didn’t start during ~60 seconds, I get IO error with code #2038 for these files.

Could it be flash error, or only server-side script can cause this problem?