Remote Procedure Call Failed

Hello, I am building a login system for my website but I keep experiencing an error on a specific PHP page that I have never encountered before and it seems worryingly low-level! It says: "The remote procedure call failed." And that's it! Nothing else is on the page. Sometimes I get a slightly different (but equally scary) error: "PHP has encountered a Stack overflow" I've googled these errors and some people have posted them but I found no solution. I've searched the PHP bug database but found nothing. Has anyone else encountered this and is there a solution? FYI: Other (more simple) PHP scripts work fine (so my PHP installation seems to be ok) and this error only started happening last week. I haven't changed this PHP/MySQL installation in months. I'm testing my script on a (Windows) server in my office on which this error occurs. When I upload it to a second remote (Linux) test server it works fine! Office Server: Windows 2000 Server + IIS 5.1, PHP Version 4.3.9 (Zend Engine v1.3.0), MySQL 3.23.49. Remote Server: Linux + Apache, PHP Version 4.1.2 (Zend Engine v1.1.1), MySQL 3.23.39. Any ideas gratefully received! Thanks, Alex