I've been running an IPN script (it's a PERL script that updates a .htpasswd file) from this credit card processing company for years and just yesterday it suddenly couldn't connect to my server and keeps getting connection time outs.
The error message shown on the credit card processor's control panel is: Unexpected Content Returned.
According to the credit card processor, it looks like the server does not allow us to access the script. They also claim that they do not have any problems with their other customers for reaching the scripts on their server and can see that the URL for the script is setup properly for my website.
However, my dedicated server host tells me that I am not explicitly blocking that IP through my hosts.allow or hosts.deny files, and I'm not blocking them with a firewall either.
I'm at wits end here because both sides are not extremely helpful and I'm losing my only source of income for this site. Any help or advice is very much appreciated!
The only way to find out is by debugging your script.
Might start by simulating the posts that Paypal sends on it, and test the responses/output, checking for any db errors. Make sure your subscr_signup subscr_cancel txn_type responses return, and update your db as you would expect.
Check that your Paypal IPN has not been disabled for any odd reason, I had this happen to me recently when the IPN URL on file could not reach the specified URL, you can also check the IPN history to see if posts are reaching your server (newish IPN feature from Paypal)
And you have probably already done this, but make sure that you can access the IPN file in your browser, if you can't read it due to an htaccess password or something like that, then Paypal won't be able to either (unless the script specifically logs into an htaccess secure area)
Anyway, basic level trouble-shooting to help you determine if it's a scripting issue or something else.