I think this may be your storage limit that is being reached...

is this possible?

or... is the 18th song a BIG file? maybe is bigger than the others and is exhaisting the max_upload_filesize limit..

I have seen this before on some of the other streaming apps I have used. Typically it is a setup issue with the end users webserver configuration. If you are using php to upload large files or batches of files at once, it can very well be your php configuration on your webserver.

Typically these are the areas within the php.ini file that can cause this issue if they are set to low:

upload_max_filesize
post_max_size

also I find upping some of the resource settings will make a big difference as well, especially with stuff like playlist and streaming or anything that is script intensive:

max_execution_time
max_input_time
memory_limit

Increasing these numbers will make a huge difference. The defaults are just to sparse for a multimedia application.