Still working ASP.NET based projects at the moment (and recovering from another Hard Drive crash and subsequent rebuild), I came across this tricky issue...
I've been working on one particular project that ultimately would be published to an external ISP. Development of the project has been pretty smooth with various 'plug-in' elements linking into the project extremely easily. However, when publishing the web site to the external ISP - certain webforms would no longer submit, using either an image submit button OR a HTML Submit button.
As you can guess, the webforms worked fine on our internal development server and on our local development machine's IIS. However, as soon the pages were published to the ISP - the offending forms wouldn't work.
After spending a couple of hours trying different things (and smoking countless cigarettes) I found the source of the problem. Those webforms that implemented any FieldValidator and ValidationSummary controls would no longer submit. And interestingly, this problem only occured in IE (I was using IE 6) whereas the mobile Internet Explorer (on a PDA), Netscape and Firefox worked fine. With a better handle on the actual problem I found this Bug Report in the Microsoft Support site. Interestingly, an FTP replace of the 'aspnet_client' directory on the remote site didn't work. And I'm sure you can only imagine the fun trying to explain the above to your ISP (those call centre has been outsourced to the far east).
Was there an alternative solution? Well, reviewing the BUG report and the output of the aspx page indicated a problem with the client-side validation code (which still worked). So, by changing all the validation controls 'EnableClientScript' to false solved the problem nicely apart from the run-trip back to the server...
HTH - M